My view (which is probably wrong)
I generaly think of ARM as dedicated systems for a specific purpose and in the case of mobiles a relativatly short operational life. Once the ARM device has been built and shipped there is very little reason or incentive to upgrade the software. It has (hopefully) all the features it needs to do its job.
Linux on the other hand as a general purpose platform is constantly evolving to add new features or to re-engineer for security or optimisation. I would have thought it would be easier to do that with a large eco system that promotes compatability.
Just my view...