Purely speculation here, but I would think that devs would want to invest their time in established gaming platforms with established customer bases. Native applications with large amounts of engine / tooling built around them.

Again shooting from the hip here, but given the restricted environment of the web browser, it would seem difficult to believe that you could make a high performing full game given that you need to download THE ENTIRE GAME essentially every single time you play, no? So that restricts the size / complexity I would wager.