The kernel has changed enormously. Vast swathes of low level functionality simply will not work without heavy kernel hacking. Many of the Nokia n900 kernel drivers simply will not work, or maybe even build, on a more modern kernel. Much of the work might be to put back old kernel interfaces, or even bugs, for userspace compatibility too. Selecting the best kernel to use is a very difficult task.

[OT: N9 stuck on .32, because that was absolutely current when we were working on it. Early N9 work was done on .29, as .32 didn't even exist yet. I tried to persuade them to let me move forwards to .33, as that was also LTS, but, even though the .29->.32 migration was described as the cleanest kernel migration ever in any of the projects, they were too scared to permit another one, as we were doing too many invasive things with the kernel, and they were risk-averse.]