The monoculture of meritocracy

The issue of which platform to use is mostly concerned with what most of the potential contributors are likely to know and which platform is covered best by most of the available documentation and shared knowledge, so if it's easier to find people who know how to write a Linux driver for some hardware, then a project will choose Linux first and foremost. That hasn't stopped other operating systems being ported to such devices, though:

http://wiki.openmoko.org/wiki/Category:Distributions

And it's perfectly possible that other platforms can take advantage of the work done by Linux developers to get driver support for hardware even if they refuse to port the actual drivers. I'm aware of at least one "from scratch" operating system project for the Ben NanoNote that has presumably been able to take advantage of the fully documented hardware and Free Software drivers already written for Linux, so it's not as if the Linux community is denying others opportunities. In fact, it's very much the opposite of that.