Possible canon reasons why the Ancients from Stargate won't interfere on the mortal plane:
1) The most powerful or influential Ancients have become so disconnected from mortals that they're basically assholes.
2) The most powerful or influential Ancients were basically assholes in real life, and stayed that way after ascending.
3) The Ancients can't act without risking alerting the Ori to the Milky Way galaxy -- which was a bad think when Jackson and Vala did it.
4) The Ancients, like the Asgard, have an enemy that is taking up so much of their time that they can't help. Maybe they have their own equivalent of Dormammu, Cthulhu, or the Outsiders from the Dresden-verse, and they can't share that because knowledge of them existing opens the door to bad things.
5) It's complicated, and some of the reasons can't be understood by mortals. This is kind of a cop-out answer, but may make the most logical sense if you consider ascended beings to be alien in the truest meaning of the term.
I always took it to be kind of a mix between 3 and 5, but 1 is an easy misinterpretation of the reality.