Anderson didn't want to spend so much time in Vancouver, away from his family, anymore, so got himself written out of the regular cast. First he was promoted to head of the SGC, and then later replaced Hammond again in his DC job (planetary defense, or something like that) -- either that or he left the service entirely so he could fraternize with Carter.
Frankly, they should have ended SG-1 there. They could have sent Daniel Jackson to Atlantis and saved the Ori plot line for when they got tired of the Wraith.
Naa they can take the wraith for another four seasons (especially now that they have made them somewhat feudal while fighting for resources). By that time Atlantis will be in 6 and it will be time for the StarGate series to end for a while..