Fragmentation is the enemy of any operating system. When you have a wide spread in adoption, you end up with a problem for both the company (resources spent supporting multiple OSes) and developers (the more platforms you have to support, the higher your costs, the tougher to make money). Fragmentation is also the enemy of security. Hard to prevent malware when your users aren’t keeping up with your latest patches.

90% is a huge adoption rate. Android has some catching up to do here.