A few things. The Rams and Raiders did not leave LA because of a lack of fans. There are still, 20 years later, probably more Rams and Raiders fans in SoCal than in St. Louis and Oakland respectively (Facebook and marketing data verify this). The Rams left because Georgia Frontierre wanted to return to her hometown of St. Louis and the Raiders left because LA would not build a new stadium with luxury suites for Al Davis. Except for the new Sacramento Kings arena, generally speaking, Cali doesn't subsidize sports stadiums (nor should they, being one of the worlds top ten economies).
Yes there are a lot of other things to do in LA, but there are also 18 million people. Filling a 75k seat stadium 16 times per year (assuming two teams) will not be challenging unless the teams are absolute bumblefucks like the Browns, Jaguars, Etc. even when they suck, the Dodgers and Angels average well over 30k, the Lakers, clippers, and kings sellout every night, and the ducks usually draw over 80% capacity for a sport that is far less popular in Orange County than football.