While history doesn't favor the odds, it is possible for teams other than Alabama, Clemson, Ohio State and Oklahoma to compete in the College Football Playoff.
Oregon and Washington have done it. LSU won it all in 2020.
So why not USC? Iowa State? Texas A&M?
It's not complicated, at least for Power 5 teams (and starting with any team that has a realistic chance to win the conference), but it's far from easy to garner serious consideration. The typical playoff path: win a conference title, knock off a handful of ranked opponents, beat a respectable Power 5 nonconference opponent and finish undefeated or with one loss.
Even then it's not guaranteed -- one team is always left out.
The sheer difficulty of winning every week, combined with the varying opinions in a subjective system, plus some scheduling luck -- teams can only hope the selection committee thinks their opponents are top-25 material -- are why only a handful of elite teams have appeared in the top four. Four teams -- Alabama and Clemson at six apiece, and Ohio State and Oklahoma at four each -- account for 20 of 28 semifinal appearances.