[MichaelMace]

Windows was already established as the desktop OS standard before Linux became prominent. Windows had a huge base of apps and massive network effects. Saving money on the cost of the OS was not enough of an advantage to make most users switch away from Windows.

Mobile is very different. There is no single established OS standard, and in fact most phone buyers don't even know what the OS is in their phone (I am talking about average users, not the folks who are reading this comment). Cellphone companies will sweat blood in order to cut ten cents out of the parts cost of a phone. Eliminating ten dollars in cost is a huge deal to them.

Add to that the marketing power of the Google brand, and Android looked very attractive to mobile phone manufacturers when compared to Windows Mobile and Symbian, which were the other main choices at the time.