I have often thought we would have avoided a lot of headaches if we started with NetBSD instead of Linux....
The reality is that in 2005, Linux had the most momentum with the silicon vendors, the largest base of developers (if you're looking to hire systems folks with experience with the kernel), and was easier to explain to carriers, OEMs, silicon vendors, investors, etc. The hardware platform was no longer highly constrained: at Danger in 2000, we started with a 24MHz ARM7, with no MMU, 8K unified cache, 16MB ram, 4MB flash -- at Android in 2005, a 200MHz 64MB ARM9 based system was the baseline -- we had a MMU! luxury! -- Linux no longer seemed like overkill for the hardware at hand.
Linux also draws a bright line between kernel and userspace with the explicit carve-out in the COPYING file declaring that userspace code need to be GPL to interface with the kernel. Explaining that "kernel is GPLv2, userspace is Apache2/BSD/MIT" is pretty straightforward. Explaining that everyone benefits from having a better kernel but the kernel is seldom what makes one product win or lose in the market is also pretty straightforward (seriously -- users only know the kernel exists if it's not working, otherwise it's not what they care about at all).