The 4K elephant in the room
Modern houses are small - modern flats are tiny.
While you can fit a normal sized HDTV in a room, once you get to the size of screen needed to gain any benefit from 4K: with double the number of pixels in each direction, there aren't that many places in yer average sitting room where you can put it. And if you want to sit at a comfortable viewing distance (which increases with screen size) - fewer still.
A 4-foot wide 55 inch telly dominates a modern living room. Given that you have one wall taken up with windows, an adjacent one with a door slapped somewhere near the middle and need to have your seats opposite the TV - there aren't that many layout options available. Put in a 60-incher and you find that the TV dominates the room. Go larger and the whole thing looks like a caricature. Stick with a 4K TV that's the same size as your existing HD kit and what have you gained for all the extra cost?
(and they still only show the same old crappy programmes)