I don’t think it accurate to say the NFL is popular and has a salary cap therefore salary caps are popular.
The NFL’s dominance, both real and imagined, stems from many factors unrelated to salary caps. Plus, there are short term and long term signs of demand waning for the NFL.
Salary caps have not helped the Browns, Lions, Jaguars, Cardinals, Texans, Bears or Jets much in the last 30 years.