I use Linux because it's a flexible, elegant and simple architecture, battle-tested under the most demanding conditions and based on a standard that has almost 40 years of service. Also because it closes any impedance mismatch between my desktop, my phone and the servers that run the code I write.

About that, as far as the two markets I am most concerned, phones (we are talking about them, after all) and servers (where I work), Linux already took over the market.

WP7, OTOH, was written from scratch because it's predecessor (WinCE/WinMO) was so miserably broken Microsoft really had no choice but starting from scratch.