My take is that they're trying to sell the game to people who haven't already purchased CS:1, or who haven't purchased any DLCs from CS:1. If you've already purchased DLC's, you've already served your purpose to the company.
Seems counter intuitive. If that was the case then the true would be of all the Sims games. I bet the majority of buyers will be from CS:1. The market audience is only so big.