I guess they overlooked the complexity of writing and maintaining a new OS. Even Android uses the Linux kernel.
I think now days it has less to do with the complexity and more to do with the fact that if you are not writing a research OS, then there are few reasons to reinvent the wheel, as it gains you very little. the Linux kernel, given it linage of supporting older, slower hardware, needs little adaptation to target it for lower powered mobile devices and it is mature. Given that the kernel is under the hood, it makes sense to use an off the shelf one that workes and focus development efforts on UI, workflow and ease of development for the platform. These are the areas where competition is still fierce and distinctions can be made. Many users will not be affected by the fact that the kernel is Linux, BSD or custom built kernel. Given that fact, it makes sense to leverage the momentum that is behind existing efforts and differentiate your product where the decision making process is happening for consumers.