It was cancelled because well, the ratings were horrible. Why? Because The CW moved it to a new night. Dawn killed 7th Heaven just like she did Reba.
Really though, 7th Heaven should have ended last season. I have never watched the show but the fact that it came back after ending is just stupid, and now its going out with it's lowest ratings ever.
Really I don't get what The CW is trying to do, they say they want this demo but then go and kill The WB's #1 comedy & strongest show? Really I don't think anything can save The CW now, almost everything is down from last year even though they have more coverage. They need a bunch of new shows fast.