Because his friends asked him to.

“You have two treatises [by me] which contain everything to do with critical days and crises—for I’m saying this to you, my friends, who compelled me against my better judgment to write commentaries on the works of Hippocrates. Keep in mind that I also wrote those works (sc. on critical days and crises) not intending that they would be distributed, but that they would be for you alone, but they happened to get out and into the hands of many people, just like many others I’ve written for you. That’s why I preferred not to interpret any of Hippocrates’ books in written commentaries. For whatever one needs to learn from him that is useful for the art has been written down by me in many treatises along with the appropriate interpretations [of Hippocrates].

“But since some of the passages that were not expressed very clearly [in Hippocrates’ writings] received a bad interpretation, so that none of those who wrote commentaries satisfied you, and you thought I was better than they were at figuring out Hippocrates’ thought, for this reason you asked me to provide for you in writing what you heard me say in my lectures. And I said the same thing to you earlier, that the interpretations will necessarily be uneven, since I won’t be interpreting all the passages equally, but I will speak more thoroughly about those that I haven’t mentioned in any of my other treatises, while I will speak in summary about those I’ve already gone through in detail elsewhere, so that I’m not forced to write about the same things many times.”

Δύο πραγματείας ἔχετε (πρὸς ὑμᾶς γὰρ λέγω τοῦτο τοὺς ἑταίρους, ὅσοι κατηναγκάσατέ με μὴ προῃρημένον ἐξηγήσεις γράψαι τῶν Ἱπποκράτους συγγραμμάτων), ἐν αἷς ἅπαντα περί τε κρισίμων ἡμερῶν εἴρηται καὶ κρίσεων. ἴστε δ’ ὅτι καὶ αὐτὰς οὐχ ὡς ἐκδοθησομένας, ἀλλ’ ὡς παρ’ ὑμῖν μόνοις ἐσομένας ἔγραψα. συνέβη δ’ ἐκπεσεῖν αὐτὰς καὶ παρὰ πολλοῖς εἶναι, καθάπερ καὶ ἄλλα πολλὰ τῶν ὑμῖν γενομένων. ὅθεν οὐδ’ ἐξηγεῖσθαι προῃρούμην ἐν ὑπομνήμασιν οὐδὲν τῶν Ἱπποκράτους βιβλίων. ὅσα γὰρ εἰς τὴν τέχνην χρήσιμα παρ’ αὐτοῦ μαθεῖν ἔδει, γέγραπταί μοι κατὰ πολλὰς πραγματείας ἅμα ταῖς οἰκείαις ἐξηγήσεσιν.

ἐπεὶ δ’ ἔνιαι τῶν λέξεων ἀσαφέστερον εἰρημέναι μοχθηρᾶς ἐξηγήσεως ἔτυχον, ὡς ἀρέσκειν ὑμῖν μηδένα τῶν γραψάντων ὑπομνήματα, βέλτιον δὲ αὐτῶν στοχάσασθαι τῆς Ἱπποκράτους γνώμης ἐδόκουν ὑμῖν ἐγώ, διὰ τοῦτό με καὶ διὰ γραμμάτων ἠξιώσατε, παρασχεῖν ὑμῖν, ἅπερ ἐν ταῖς διὰ λόγων συνουσίαις ἠκούσατε. κἀγὼ τοῦτ’ αὐτὸ προεῖπον ὑμῖν, ὡς ἀναγκαῖον ἔσται τὰς ἐξηγήσεις ἀνωμάλους ἔσεσθαι μὴ πάσας ὁμοίως ἐξηγουμένου μου τὰς λέξεις, ἀλλὰ τελεώτερον μέν, ὑπὲρ ὧν οὐδαμόθι τῶν ἐμῶν πραγματειῶν ἐμνημόνευσα, διὰ κεφαλαίων δέ, περὶ ὧν ἤδη τελέως ἐν ἐκείναις διῆλθον, ἵνα μὴ πολλάκις ὑπὲρ τῶν αὐτῶν πραγμάτων ἀναγκάζωμαι γράφειν.

Gal. Hipp. Prog. 3.6, 18B.229–231 K. = 328,4–22 Heeg

Because he would be criticized if he didn’t.

“Well, let them say these things, but let them not falsely accuse Herophilus, nor intimidate with a sacred name those who are ignorant of Herophilus’ writings, nor use it to contrive trust in their argument. For it is shameful to dispute something on the basis of witnesses as if one were in a court of law. If you are able to argue towards a demonstration, I’ll gladly listen to you, but to call on Herophilus and the Herophileans as witnesses is to run away from a direct dispute and argument, and for fear of refutation invent evasions and deceptions.

“Clearly, false witnesses are brought in for the sake of a story, not in order to inquire into the matter further. ‘Does Herophilus say this?’ ‘Certainly not.’ ‘But doesn’t that mean you are lying?’ ‘Why don’t you show that I’m lying, show what he says.’ And then some kind of ambiguous passage is presented and there is a fight about what the passage says and what it means. Suddenly, rhetorical exercises on language and meaning are being rolled out, then others on ambiguity and, by Zeus, on inference. But they even apply the whole theory of rhetoric, as if they were practicing a speech, not bringing the subject to a conclusion. For what hasn’t been said by modern doctors about this question, some maintaining that Herophilus also knows this differentia of the pulse, others trying to show that he does not know it?

“Well, both groups have had a hard life and deserve to be pitied, the first for their ignorance, the second for their competitiveness. But we also have a hard life, since it is not enough for us to practice the proper theoretical study of the art, but we want to know what Herophilus said, what Heraclides, Chrysermus, and Hegetor interpreted incorrectly, and what Apollonius and Bacchius and Aristoxenus said. And even if we do not want to, at any rate we are forced to do it and we get to enjoy both troubles: that we get involved in this nonsense when it is unnecessary and that we unwillingly act like they do. Now it turns out that I must do one of two things: either appear to say the opposite of Archigenes, Herophilus and countless others, or show that there is no full pulse according to Herophilus.

“If, therefore, I had chosen this task, I would be just as blameworthy as they are for the vanity of the study; however, as the case stands, having refuted their opinion according to the actual truth of the matter, I will let those people leave who are interested in the practice of the art itself, since we’ve already completed our inquiry into the subject. Anyone who wants and has time to learn about ancient history, I will add the whole thing, showing that Herophilus does not use the term ‘the full pulse’ anywhere for anything.”

ταῦτ’ οὖν αὐτοὶ μὲν λεγέτωσαν, Ἡροφίλου δὲ μὴ καταψευδέσθωσαν, μηδὲ δυσωπείτωσαν ὀνόματι σεμνῷ τοὺς ἀμαθεῖς τῶν Ἡροφίλου γραμμάτων, μηδ’ ἐκ τούτου τὴν πίστιν τῷ λόγῳ ποριζέσθωσαν. αἰσχρὸν γὰρ ἐπὶ μαρτύρων ἀγωνίζεσθαι, καθάπερ ἐν δικαστηρίῳ. εἰ λέγειν ἔχεις εἰς ἀπόδειξιν, ἡδέως ἀκουσόμεθά σου. τὸ δ’ Ἡρόφιλόν τε καὶ Ἡροφιλείους καλεῖν μάρτυρας, ἀποδιδράσκοντός ἐστι τὸν ἐξ εὐθείας ἀγῶνα καὶ λόγον καὶ διαδύσεις τε καὶ μηχανὰς ἐξευρίσκοντος ἐλέγχου φόβῳ.

δῆλον, ὡς ἕνεκα τοῦ μὴ περὶ πράγματος ἔτι ζητεῖν, ἀλλ’ ἱστορίας οἱ κατεψευσμένοι μάρτυρες ἐπεισάγονται. λέγει τοῦθ’ Ἡρόφιλος; οὐ μὲν οὖν. ἀλλὰ οὐδὲ ψεύσῃ; δεῖξον πῶς ψεύδομαι, δεῖξον πῶς λέγει. κᾄπειτα λέξις, εἰ οὕτως ἔτυχεν, ἀμφίβολος προβάλλεται καὶ πόλεμος ἄμφ’ αὐτῇ συνίσταται, τί ποθ’ ἡ λέξις λέγει καὶ τί ποτε βούλεται, καὶ κατὰ ῥητὸν ἤδη καὶ διάνοιαν ἐπιχειρήματα ῥητορικὰ καλινδεῖται, καὶ κατ’ ἀμφιβολίαν ἕτερα καὶ κατὰ συλλογισμοὺς νὴ Δία. ἀλλὰ καὶ πᾶσαν οὕτω μεταφέρουσιν τὴν ῥητορικὴν, ὥσπερ ὑπόθεσιν μελετῶντες, οὐ τὸ προκείμενον περαίνοντες. τί γὰρ οὐκ εἴρηται τοῖς νεωτέροις ἰατροῖς εἰς τὸ πρόβλημα, τοῖς μὲν κατασκευάζουσιν ἐπίστασθαι τὸν Ἡρόφιλον καὶ ταύτην τοῦ σφυγμοῦ τὴν διαφορὰν, τοῖς δ’ ὡς οὐκ οἶδεν ἐγχειροῦσι δεικνύειν;

ἀταλαίπωροι μὲν οὖν ἑκάτεροι καὶ ἐλεεῖσθαι δίκαιοι, τῆς μὲν ἀμαθείας οἱ πρότεροι, τῆς φιλονεικίας δ’ οἱ δεύτεροι. ἀταλαίπωροι δὲ καὶ ἡμεῖς, οἷς γε οὐκ ἀρκεῖ τὴν ἰδίαν ἀσκεῖν θεωρίαν τῆς τέχνης, ἀλλὰ τί μὲν Ἡρόφιλος εἶπεν, τί δ’ Ἡρακλείδης τε καὶ Χρύσερμος καὶ Ἡγήτωρ οὐκ ὀρθῶς ἐξηγήσαντο, τί δ’ ἂν εἶπεν Ἀπολλώνιός τε καὶ Βακχεῖος καὶ Ἀριστόξενος εἰδέναι βουλόμεθα. καὶ εἰ μὴ βουλόμεθα δὲ, πάντως ἀναγκαζόμεθα καὶ διττῶν ἀπολαύομεν κακῶν, ὅτι τε φλυαροῦμεν οὐδὲν δέον ὅτι τε μὴ βουλόμενοι τοῦτο δρῶμεν, ὥσπερ ἐκεῖνοι· νῦν γοῦν ἐμὲ δεῖ δυοῖν θάτερον, ἢ δοκεῖν Ἀρχιγένει τε καὶ Ἡροφίλῳ καὶ μυρίοις ἄλλοις τἀναντία λέγειν, ἢ δεικνύειν, ὅτι καθ’ Ἡρόφιλον οὐδείς ἐστι πλήρης σφυγμός.

εἰ μὲν οὖν ὡς ἔργον αὐτὸ προὐχειρισάμην, ἦν ἂν ὁμοίως ἐκείνοις ἄξιος μέμψεως ἐπὶ τῇ ματαίᾳ σπουδῇ· νυνὶ δὲ κατ’ αὐτὴν τῶν πραγμάτων τὴν ἀλήθειαν ἐξελέγξας αὐτῶν τὴν δόξαν, τοὺς μὲν πρὸς αὐτὰ τὰ ἔργα τῆς τέχνης σπεύδοντας ἀπολύσω πρὸς ἐκεῖνα, συντετελεσμένης ἡμῖν ἤδη τῆς προκειμένης ζητήσεως. ὅσοι δὲ καὶ ἱστορίαν ἐκμανθάνειν παλαιὰν ἐθέλουσι, καὶ χρόνον εἰς τοῦτ’ ἔχουσιν, ἅπαντα προσθήσω, δεικνὺς ὅτι μηδαμοῦ χρῆται πρὸς μηδὲν Ἡρόφιλος τῷ πλήρει σφυγμῷ.

Gal. Dig. Puls. 4.3, 8.954–956 K.