As others have said, a lot of people already own iPads and there's nothing about the current concept/design that encourages especially upgrade cycles. Not that there's anything wrong with that from a consumer perspective. It does what it does pretty well and doesn't really call out for incremental improvements to battery life/resolution/processor power/etc. although those do naturally come about with each iteration. And, of course, the carriers don't subsidize upgrade cycles as they do with phones.

Being able to share data plans with phones at nominal cost would be one interesting upgrade driver but that's really in the hands of the carriers and is unlikely.

The other interesting possibility across both Macbook and iPad lines would be some sort of reconvergence of tablet and laptop experience but Apple got to where it is by doing the precise opposite so I'm not sure Apple feels a big impetus to shift up the playing field right now. (Microsoft OTOH has been trying to parlay its desktop strength with tablets that are more tightly coupled but with somewhat limited success--to say nothing of the fact that Windows 8 was largely a product of this thinking.)