Ryan Field seats over 47,000. ND, which hasn't played in Evanston since 1976, would almost certainly sell the place out. Wrigley has a smaller capacity. And the whole point of NU playing there is as a gimmick to increase attendance, which wouldn't be necessary when ND is the opponent.
If the 2018 game isn't in Evanston, the more suitable alternative would be Soldier Field, where the 1992 and 1994 games were played.