The real draw of Android and iOS, for developers and publishers, is the gigantic userbase that already exists there because those are super cool multipurpose devices people buy for countless reasons.
The popularity of these platforms has little to do with the quality or even the price of games available: the games are extras and people would still buy these devices in droves even if they didn't have any games. This is why low budget games for such platforms are both low risk and bring big returns: a large amount of people actually like playing games, but only a fraction like it enough to "commit" to it in some form, be it buying a dedicated device, buying a controller for their multipurpose device, buying a paid game, buying IAP for a free game or even talking about games with their peers.
When you remove mobile/tablet games from their environment and put them into a box whose main purpose is gaming (a device that hooks up to a TV and has a game controller as primary input device), they lose the advantage of a preexisting audience that "might as well play a game if it isn't too much trouble" because now there *is* trouble: you'll have a controller in your living room and a device hooked to your TV and wall socket for everyone to see, something very different from the dirty hidden folder full of "dirty" games in your phone or a bunch of Flash games in your computer bookmark.
It's like porn. Tons of people browse porn in their phones but only a fraction subscribe to porn cable channels.