The Los Angeles Coliseum is too old for the NFL, with the current emphasis on premium seating and modern facilities. A new stadium costing a fortune is a given.
Besides, USC football averages over 85,000 a game. Who is going to give up watching an elite College team for NFL tickets ?
It's been 10 years since LA had a team and there's a legit reason for the absence. They don't support NFL football.