Why have so many rival professional sport leagues failed in the US?

Many rival professional sports leagues in the U.S. have failed due to a lack of market penetration and brand recognition compared to established leagues like the NFL, NBA, and MLB. These established leagues benefit from deep-rooted fan loyalty, extensive media contracts, and significant financial backing, making it challenging for newcomers to attract audiences and sponsors. Additionally, many leagues struggle with inconsistent quality of play and operational sustainability, leading to diminished interest and support.