I would say sports just moves faster than academics. The average sports career is only a few years with a few exceptions, the average academic career is much longer.
That's true and, furthermore, a big part of why a university is well-ranked in year N boils down to it being well-ranked in year N-1 and N-2. With a sports team, if you have some critical injuries or otherwise lose important personnel from the prior year or two, no one cares that you won the championship a couple years back if you have a mediocre record this year. There's just a lot more inertia in university rankings both on objective factors and on things that basically depend on being a good university because they're a good university.
The same could be said of students. Universities are made or broken by their recruitment, coaching/professors, administrators, and facilities. Sports may be slightly more volatile, but I doubt it’s nearly as much as people assume.