The problem is that Linux's kernel interface isn't stable, so a module written for one kernel release won't work on another, and (AFAIK) they don't do major-minor releases (like PHP,Apache) where, say, the 4.0 kernel is stable and will stay stable for a few years. Rather, 4.1 could introduce braking changes as far as modules go.

And, unlike PCs, most phones don't have standard hardware, so someone kernel modules tend to be closed source and don't have "basic" mode.

As a result, to upgrade to a new Android release someone has to backport all new (kernel) features to old kernels.