My understanding is that a number of Android vendors operate by just taking a version of the Linux kernel and making any changes they need. By the time the phone is released and anyone cares, it's an old version of the Linux kernel, and the changes weren't written in a way that it's easy to incorporate them in the first place. The idea of incorporating a large chunk of changes all at once at the end is nobody's idea of a good time.

I get that they have no responsibility vis a vis the GPL to upstream their changes, and I have no doubt that this is a cheaper way for them to run their business.