I've played quite a few different Dynasties this year and taken some over 15 years. I have noticed that invites tend to depend on who I play in my out of conference games. I also have to beat them I think. I'm not saying this will assure you an invite but I beleive it helps. I know everytime I have gotten an invite, the conference that invites me has a team that I had beaten that same year in an out of conference game.
I would guess it could have something to do with the records of the existing teams in that conference too. I mean you wouldn't expect a conference to kick a team out if the whole conference is playing good. Though I'm not sure if this EA game is that robust.