Quote from JetsetlemmingCould be pressure from retailers or maybe the margin for Activision is higher when the copy is sold in a brick and mortar store than when distributed over Steam or...
There's no way on Earth Valve takes a bigger slice than Walmart + the cost of physically printing and distributing copies.
*shrug* it could have many reasons, maybe Activision wants to keep Valve (a competitor) away from the day 1 sales.