I think the league that Herschel Walker and Jim Kelly played in had limited success and some others liked it like I believe Al Davis who thought Summer ball was a good idea.
I’m not sure why they folded but enterprises like that need to have a sound approach. I think they played some good ball.
It seems San Antonio was excited about having a pro-team with the NFL continually denying them a franchise.
Summer football had some serious problems, namely sunbelt June temperatures are not conducive to wearing football pads.
The USFL folded because they ran out of money. They limped along OK for a while but then they changed the overall strategy of how to operate the league. The new strategy was more expensive and they either needed to merge with the NFL or get a big settlement on the anti-trust suit against the NFL when they didn’t get it that was the end of the league.