That topic has been covered a million times. UND was in the playoffs and had multiple home playoff games from 2001 to 2007, with 2002 being the only year they did not reach the playoffs. Those first round games were boring matchups that were repeats from previous years (see Winona State). The fans did not care to see those same doormats every year in the first couple rounds.
Also, that was the D2 days and the hype is a weeee bit bigger now. When NDSU won the NC, it assured they would sell out every game from then on until they started losing again.