Well, they don't have any serious competition at the moment. Why push prices up or profit margins down with a high res screen when they can introduce one at a later (and cheaper) date?
Pushing prices up or margins down would make them less competitive. More importantly, they have to be able to manufacture with quality in increasingly gigantic volume. Sure they're making choices, but that's not the same as holding back.
It's also a nice trump card to pull out if a nice shiny new tablet looks like it will begin eating into Apple's marketshare. If they think the iPad 2 can already comfortably take on the Xoom (etc), why would they try to add too much now?
