Greg Kroah-Hartman: Android and the Linux kernel community

The overwhelming majority of the problem ARM faces is that most of the CPU vendors have traditionally seen Linux as a traditional embedded OS that you produce a BSP for with your own custom stuff in it and don't contribute to. This means that the core code required to boot the processor from one of these systems is non-mainline, and few of the companies building products have the time or resource to mainline it. The end result is that you end up with communities coalescing around the vendor BSPs which are often based on old kernels which make it difficult to share anything with mainline.

This is getting a lot better these days - many of the major SoC vendors are contributing their ports upstream.