The only reason why the Iphone 2G is still getting support is because it has identical hardware as the 3G, apart from 3G of course. It will be interesting to see what happens after summer; if Iphone is ever going to support multi-tasking, then the support for the older models has to be dropped. Keeping the same software for a wide range of devices either unables to make use of the new hardware, or slow down the older ones with the new power-demanding improvements - both of these cases can be experienced with the Iphone family.
As for Android, exclusivity deals is to blame. Some OEM:s, like SE (which supports various platforms unlike Motorola, which is devoted to Google nowadays), didn't have a choise but 1.x in 2009 - yet some Motorola devices had 2.0 already at the time. Actually it's SE:s fault that their prototype (X10) never became a consumer-friendly handset within reasonable time - but still, with 10s of devices announced for 2010 with older initial versions still to be released, support can't be dropped. As of the exclusivity deals, I really wonder what Google offered to satisfy all bigger players (including PC OEM:s) but Nokia, Apple, Palm and RIM.