I went to Purdue for my undergrad, so here's an honest, inside view:
-Purdue fans have a massive inferiority complex. They are behind ND in football, and IU in basketball. The Boilers don't have bragging rights in any major sport in Indiana, and it kills them.
-The Notre Dame game is 100x bigger than the season finale "Battle for the Old Oaken Bucket." Purdue fans would rather see ND lose a game than Purdue win one.
-Even the most delusional Purdue homers knew the Drew Brees era was an anomaly, and not a sign of things to come, so that adds to the bitterness.
-You have to go to a Purdue bowl game (which is rare) to see that they actually have some reasonable, decent fans. The students are obnoxious, but what do you expect with "Breakfast Club" on campus every Saturday for a home game?
*Throughout my time at Purdue I saw the good, the bad, and the ugly. Overall, I have no animosity towards them, and wish the Boilers well 11 games a year.