I think it's as simple as their great coach left. He didn't coach there during WWII, and when he came back, it wasn't the same.
When HCs return to a previous stop, it never seems to go as well as the first time, does it? John Robinson at USC, Bill Snyder at K-State. I'm sure there are plenty of other WWII examples like Bernie Bierman, but I really do just think that's it.
It's also a major indicator that a school is actually an elite program vs a one-coach peak when they have multiple elite runs under different HCs. Minnesota had that from the 1910s-1940ish, but it's simply highly unlikely to continue that forever. That's a good 30-year run of multiple peaks under multiple HCs.
Florida had a great 30-year run under multiple HCs, from the early 80s to 2012. The ebbs are fun, but the flows afterward suck. From 83-85, the Gators went 27-4-3 under Pell and then Hall. Spurrier won 102 games in the 90s. Meyer won 2 NCs. Hell, even Muschamp had us at #3 going into the bowls one year.
But that time ended, and it happened to be 70 years after Minnesota's run.
Nebraska's run was about 30 years, too. 1970-2001. Only 2 HCs, but both reached the pinnacle multiple times. Idk, maybe there's something to 30-year runs.