ICS adoption figures are still low.
But that is not the only issue. Eclair and Froyo together have more then 20% market share. These are 2.x version of android that most manufacturers have updated to 2.3 (gingerbread) so why are there so many 2.1-2.2 installation?
The answer is probably that the users do no care to update and probably do not even know that they can update.
Android does not require the user to install software on the computer to sync files and so on. It also does updates OTA (Over The Air). But because of this there is no nagging of lazy users, no popup telling you that there is a new version every time you connect the phone to the computer.
It might mean that great features that make Android easy to upgrade and connect with computers, also means that some users never update.