First, it's accessibility. If I write half-decent HTML, I'm covering Android, iOS, GNU, Windows, Mac, text-mode, audio-mode, noJSers, non-humans, and many other users and scenarios, including ones I can't even think of. All in one shot.
Also, developing for Android and iOS seems like a huge pain in the ass in comparison to developing for Web. Also, there is the red tape of the app stores and the real possibility of instantaneous (and irremediable) shutdown at the whim of some moderator.