Plenty of reasons, I’ll list a few.
*Corporate dollars will be reduced (buying suites, tickets, ads, etc)
*Regular ticket sales - some fans will opt to spend on NBA or buy some of both - further depleting revenue.
*Must share facility - limits scheduling flexibility, practice time on court where you play games, etc
Media coverage - will be reduced/split.
Many, many more reasons. I’ll argue my point further by asking a question. Name a prominent, big-time D-I program that splits time in an NBA arena and is highly successful year after year? There may be one program that could argue that - possibly Villanova.
But even Nova is not near the program Louisville has been throughout history. And Nova’s attendance (9,772 last season sharing with the 76’ers about 2/3 of the time, playing at Palestra on campus the other 1/3) and revenue pales in comparison to UofL.
This is just my brief take, I can get much deeper.