Because they get outdated far quicker than iPhones do. Where an iPhone will be the 'latest' for almost a year, an Android phone is lucky to be the latest for a few weeks...
The SGS2 is about to be outdated by the new generation of dual core Android phones...Nexus Prime, HTC Pyramid, SGS2+, LG Optimus...so the value of the current SGS2 is going down as a result...
It's just the downside of such a fast moving market...