The main problem: demographics. We need 4 years for more old people to die, and younger voters are far more likely to vote in a presidential election than a gubernatorial one [and no one's fighting to put it on the 2009 and 2011 ballots]. And apparently the tilting of opinion has stagnated for the past couple years.
Subsidiary problem: money. If we blow our [recession-depleted] wad in 2010, and lose, will we be able to regroup and spend a whole bunch more money needed to do it again in 2012?
I still think 2010 is better, and I expect that (whichever side wins) we'll have to do this at regular intervals anyway. And at least it's free to vote. But I can certainly realize that the decision to put it off til 2012 isn't idiotic. However, I do think it runs the risk of sapping away the energy that exists to overturn Prop 8.