[hombre_fatal]

I don't think that's the reason.

Touchscreens let you build arbitrary UI/UX. You can click anywhere and do gestures anywhere and type anywhere. When there doesn't need to be UI, like when watching a video, the whole phone is the screen. So the UI can optimize for the best UX. It's much more powerful.

With physical buttons, software is pigeon-holed into UI designed around those buttons. It's a massive trade-off. Something we take for granted like navigating a website becomes much more tedious when you only have buttons.

Just look how much effort goes into making software-specific hardware like the scroll wheel/drum on old-gen music players like the iPod, yet it doesn't solve something as simple as typing in a song search query.