The difference is: Android came into a brand new and exploding market where hardware vendors had pracitcally no other option. Yes Apple was very dominant in this market, but it was only one vendor among many. If those others had the chance to install iOS on their hardware, they'd have done so. But they couldn't and Android was the only option with a chance of being competitive with the dominant system. Symbian and Windows Mobile were outdated and Blackberry OS proprietary same as iOS. And they needed something quickly.
Contrast that with the situation in desktop computing. All vendors, really all of them have access to the dominant software platform. It does not even cost them much and has overwhelming customer acceptance and for the most part is even what customers explicitely want. Even Apple Inc. which is the world's most valuable company with all it's hype behind it has a hard time competing in this market with some 4.5 % market share.
Other than with smartphones there is 0 incentive for vendors (except Apple) to ship something other than Windows. There is no pressure to find or support another system.