He’s skilled in the traditional big man roles. Finishing at the rim and protecting the paint. Those skills are much harder to find in guards.
Rudy is one of the best at those skills and pretty much single handedly guarantees a good defense, but investing so much resources in an offensive liability becomes troublesome in the playoffs. Teams can construct game plans to lessen Rudy’s defensive impact. Yes, he’s become a pretty good defender on the perimeter, but the problem is that if he can be drawn out to the perimeter, no one is defending the rim.
Rudy plays a role on offense and he plays it well, but when the game is on the line and you need a bucket, he’s not an option. Mitchell might turn into a good option, but right now he’s still pretty inefficient. They are spending a lot of money on Bojan and he’s a decent option, but nothing like the Kawhi, Lebron, Lillard, Luka, etc. options that other western teams have.
A reliable, efficient, bucket getter is the Jazz primary need. Spending the super max on Rudy would make that more difficult to achieve and pretty much lock you into a 2nd round ceiling.