ESPN makes the majority of their money from cable and streaming packages like YouTube tv. They use their popularity as leverage to take a large cut from those packages. If they offered a streaming service with the premium content that is available on those packages they would lose their leverage on the cable companies.  

​https://www.reddit.com/r/cordcutters/comments/zi9rcz/so_just_to_get_this_straight_espn_cannot_let_us/