It is obvious when you compare the résumés of Clemson and No. 3 Alabama. Neither the Tigers nor the Crimson Tide have beaten a team ranked in the top 25. They share the same "best" win: Texas A&M. Clemson has beaten more Power 5 teams than Alabama has. Clemson has beaten more teams with winning records than Alabama has. Clemson's opponents have a better overall win percentage than Alabama's.
Nothing, it seems, helps Clemson: Not a better schedule than Alabama's to this point. Not a win margin of 29.4 points (that includes the one-point win over North Carolina). Not seven games in which it has scored 40 or more points. Not a 24-game winning streak or its title as defending champion. Not even the advanced metrics, such as FPI and SP+, which both rate Clemson higher than Penn State.