Why would Fox renew "Doll House" and not "Terminator: The Sarah Connor Chronicles"?

I agree, but a lot of it is financially motivated. Warner owns the rights to the Terminator franchise. They allowed Fox a discount price to use their property for TSCC for two years, mostly to draw attention to the movie. That agreement has ended, so Fox would have to pay more for the rights next season. Also, the show is expensive to produce. This is a bad combination given the ratings and the economy.

Dollhouse, on the other hand, is owned by Fox. They produce it in-house, which means it's cheaper. Whedon reportedly agreed to serious budget cuts for season 2 as well. Of course, TSCC would have brought in bigger numbers due to the movie, and Dollhouse's ratings started out bad and sunk like a stone. It probably won't last long. But it's less of a risk for them financially.