Because in some way this boils down to the original question people asked about the 770 when in was introduced years ago: "What is it? And: Why?!".
To me, the tablet does replace the desktop PC when I'm not at home (or too lazy to go to the other room). In fact, it's faster and more powerful than some of the PCs I work on every now and then.
Giving the tablet the same functionality would simply duplicate my phone, only much bigger. I don't have use for two devices that do the same, so I'd go and sell my tablet then.
Maybe there shouldn't be a "UI for the tablets". There should be a UI for using the tablet as a mobile browsing device (=the "phone-use-case") and a different one for using the tablet as a micro-laptop (=the way I use it).