They've done this before; the ATV1 didn't support 1080p, at a time when 1080p TVs were starting to exist in the semi-mainstream (though the vast majority of the market was 720p or 1080i still), but there was little content for them.

Ultimately, it'd presumably add cost for a feature useful to few people. Connectivity for 4k also remains a mess; there's HDMI 1.4 (24hz at 4k, so not good enough; the interface presumably runs at 60fps like all other Apple interfaces), and HDMI 2.0, which does support 60hz at 4k, but which is somewhat rare, and has its own problems: http://www.avsforum.com/forum/168-hdmi-q-one-connector-world...

I expect in a couple of years when 4k TVs are more common, content exists for them in significant amounts, and the connector standard (possibly HDMI 2.0a?) is sorted out, they'll support it.