IMO, that's because Samsung doesn't have a clue and would rather hedge their bets. Most people I know who has a Samsung Galaxy, Captivate, etc have complained endlessly about the wait for updates.
Samsung's business is in selling phones, not developing a software platform and therein lies the problem. They make their bread and butter on planned obsolescence and in the process screw over their customers.
But this points out a potential weakness with the model of taking someone else's OS and slapping it in your device.
Once the operators (typically) have added their crap on top it leaves you little margin to extract extra cash until you get someone to buy another device.
Seems to work alright for Apple - even though they're doing gangbusters on App sales, it seems the majority of iPhone's profitability comes from hardware sales themselves, and this is without any real shades of planned obsolescence.
Nokia is renowned for their hardware build quality - well in advance of LG, Moto, or Samsung. I think the market has spoken that they want the full experience - solid software, beautiful hardware, and impeccable integration of the two. Samsung IMHO has fumbled that ball up until now, producing cheap-y hardware coupled with lazy software integration. There's a lot of room for someone who knows what they're doing with Android to come and eat their lunch.
Samsung's business is in selling phones, not developing a software platform and therein lies the problem. They make their bread and butter on planned obsolescence and in the process screw over their customers.