In a 12 game regular season, ND would have played only 2 more games, likely winning both (AS AN ACC MEMBER). They already had the strongest 1-loss record, which is why they made it in the first place. When you look at the rest of the field, the likelihood of finishing with a stronger win slate (ND bashed UNC, and did beat Clemson whether understrength or not), is very low.
2 Conference Champs already had 2 losses
3 P5 teams had 1 loss and didn't make it:
TAMU has the strongest argument re: ND but got destroyed by Bama in the regular season. Their quality win vs. Florida was diminished by the end of the season
Indiana - they're not making the playoffs without finishing undefeated, period. They had 1 loss, so no.
USC - lost in title game to a 2 loss team - they're not making it.