It makes sense. It's a free and open source operating system for mobile devices. It's based on the Linux kernel which many developers are aware of and have experience with. It's backed by Google. Customers want it. Handset makers want to compete with the iPhone. There's a decent number of applications already built for it. There's room for customization so that each manufacturer can differentiate their Android from the other Android units. Why spend your own money developing something that's likely to be inferior?