If you ask me, it's two words...
Microsoft Cashback
The 50D was released in the midst of the cashback offers, and so many people could buy them and get $200 off. So they could buy it, try it out, and if they didn't like it sell it and lose nothing on the deal.
Also, if you remember, Canon came out with some aggressive rebates soon after the release. Remember it was released at $1399, then a month after release Canon dropped the price $100 to $1299, and then came out with a $200 rebate, dropping the price to $1099 all within the first 3 months of the camera hitting the market. Couple that with the Microsoft cashback and people were getting 50D for $900.