I think you have to realize that Northwestern, Vandy, etc. understand that they get a benefit from the bell cows doing well. ISU was in a similar position, in the Big 8, which used to be nicknamed “The Big 2 and Little 6”. ISU understood that it benefited from OU and NU doing great.
The SWC, and to a similar extent, the PAC were different. Those conferences were configured where the big schools (Texas, TAMU) considered the little schools’ locations their recruiting territory, breeding resentment at the little schools. Baylor fans, for example, resent Texas in a way foreign to ISU fans feelings about OU. Texas recruits Waco, and when Texas is doing well, Waco stores sell more Texas stuff than Baylor stuff. The small SWC schools have a shared interest in Texas being down. In the SWC, this manifested in the conference being OK with massive cheating, leaving it to the NCAA to clean up.
A similar dynamic resulted in the Big 12. ISU may not have resented Texas, but Baylor, TCU and TT sure did. ISU and OSU certainly saw the opportunity to take advantage of with Texas down, circling those dates on the schedule.
I’m sure some is Texas’ fault, primarily for allowing itself to falter, but the larger point remains- Northwestern doesn’t hate Michigan and Mississippi State doesn’t hate Alabama the way the Left Behinds hate Texas.