In my opinion, it's a few things.
First, Vanderbilt is not as invested in athletics as other major programs. They are invested to a degree, but God bless 'em, they treat sports more like a function of the college rather than a business (as the major P5 programs do). That's not necessarily even a bad thing, considering what the money has done to college sports.
Second, Vanderbilt's brand is competing academically with Stanford, and athletically with Tennessee. Vandy's academics are not as renowned as Stanford's, and its athletics program is not as renowned as Tennessee's. It is in a figurative hole that it has to dig itself out of to succeed, and every time one of their major programs loses a good coach, they go right back into that hole. And, considering their school priorities, they aren't really going to try to change that.