So you buy an iPhone.
Within one year the hardware is "obsolete". However you can still run the latest software and use the latest apps. So your 24month contract does really last well with the phone you purchased.

With Android its completely the opposite.
Phones are released weeks, if not months, after the source code for the latest android build yet you still get it with one major revision behind. The oem promises update to the latest one "soon", which normally shouldn't exceed 2 months. However in most cases there is nearly six month period until the update trickkles out, even longer before you get it. By this time, another major revision is undergone giving new features, spec bumps etc.
In most cases, after purchasing an Android device you are left behind and usually at the mercy of the developer community (CM/MIUI/Custom), and currently they only account for a small portion of the market.

If Google put in a restriction to the Google Market (and other proprietary software), OEMs would have to comply and so would the carriers. I read somewhere that over 70% of apps were written with the Android 2.1 SDK so that developers can get as much compatibility as possible. This means apps have poor performance on the latest hardware, which is one area where Apple excels.

Now a typical contract is 2 years. If Google said every Google phone has to be updated to the latest build within 1.5 years, it would really create an incentive for developers to utilize the improved sdks. And if Google said that devices must be supported for 3 years, it would again increase the minimum sdk developers use to target as much devices as possible.

Google upgrades the Android build to a major revision every 6 months (2.1-> 2.2-> 2.3). In between, there is bug fixes, security patches, added functions such as 2.3-> 2.3.3-> 2.3.4-> 2.3.5-> 2.3.7 etc etc