Because if you do, you'll be paying too much. Instead of the promised 560MSP price point, the Left 4 Dead expansion has hit the Xbox Live Marketplace at 800MSP. You might want to wait until it's fixed.
And fixed it will be, Microsoft's Larry Hryb confirming that the price is indeed a mistake, and not some exercise in misery wrought by the dark overlords of Xbox Live. In his words, "hold off until you see that price before you purchase", which should be sometime later today. Probably sometime soon.
Update: Corrections are rolling out across the world in stages. I can see the price has been corrected to 560MSP here in Australia, but you may want to double-check yourself before blindly purchasing.