Gingerbread still popular
People still wonder about the slow adoption of Android 4.x, although IMHO there is not much to wonder, nor is it much of a hassle.
The Android ecosystem is very far from homogeneous, you've got devices that are sold for $100 and $700 in it. Some of these lowend/older phones don't even bring the specs to be capable to run Android 4 officially. (Inofficially, the HTC Dream, the first Android phone can be upgraded to ICS.) So even 2012 you still can get newly released phones with Gingerbread.
Then users don't see the point of upgrading, it works, so why change anything. (Don't ask, got 2 S2 users in my home that explicitly told be keep my hands of their phones) Android is way more modular than other mobile OSes, and quite a bit of builtin functionality like Google Maps, Email, ... get just updated as apps.
Then you've got the issue that the manufacturers think that they need to include their own set of patches on top of Android, which slows down the time till an official upgrade for a device is available.
In practice, if one is willing to flash an unofficial image, most phones can be brought uptodate.
So don't expect iOS style instant update effects, a more realistic comparison would be the PC market (XP, Vista, Windows7 and Windows8 are all out in the wild nowadays), although for a number of reasons (modularity) there is less of a reason to upgrade.