I just don't think Notre Dame has the athletes to stay in this game. There isn't one area where they are bad, but I don't believe they can really compete against upper echelon competition, i.e., Alabama, Clemson, Oklahoma, Georgia and Ohio State. Yes, they beat Michigan earlier in the year, but I also think they were bit of paper tigers.
I personally didn't think Notre Dame deserved to be in the playoff because I just don't buy that they are one of the four best teams. I also don't think a team that plays their brand of football can legitimately compete over the course of potentially two games against the teams I mentioned previously.
Colin Cowherd had a pretty good discussion about this not long ago where he describes what he calls "southern teams" and "northern teams" and he doesn't believe a "northern team" can win a national title, and I tend to agree with him. The biggest difference the northern and southern teams is the quality of athletes that make-up the rosters. He considers Ohio St a southern style team given that they effectively recruit players in the south and those in the north that are elite. Teams like Wisconsin, Michigan St, Penn St, Stanford, and Notre Dame just don't have the athletes top to bottom. Michigan is trying to get there, but aren't yet.