Of course, it's all about the $.
Reports are that Fox owns ALL of the teams has committed to spending $50M/yr for 3 yrs to "operate" the league.
So, if it seems like Fox is really "pushing" the USFL by broacasting the games live (and repeatedly in replays), this is why. They have to sell as many ads as they can, as many times as they can, to recoup this "investment" (and, if they're lucky, actually make $ on it).
See:
https://www.sportskeeda.com/american-football/how-much-usfl-franchise-cost
BTW, since Fox has exclusive rights to the USFL, I doubt they'd be sharing any coverage with ESPN or other broadcast channels unless they are willing to pay for the privilege which they obvioulsy have not interest in doing since it would only benefit Fox and not them.