I used to enjoy Overwatch enough to check out a few matches of the OWL, and while I can't speak for anyone else, I had a few reasons for why I didn't care.
Firstly, the way the game is setup contributed to why I quit watching, though it was the direct cause of why I quit playing: I used to be more of a support main 'cause I liked being a team player, and Blizzard constantly gave the middle finger to support players. The camera never followed them in OWL, and it never felt rewarding to play in either QP or Ranked. Their general design and playstyle was significantly less interesting than the DPS characters, and the league was pretty obviously aware of it because they never gave a shit about support. I'm more or less over playing support characters/classes, largely thanks to Overwatch, but just about every big play you'd ever see in the league would be from the Genji player and in general the team compositions were incredibly static. It's just not fun to watch. Half the team isn't doing anything interesting, and the other half play as the same DPS heroes every single game. Oof.
Secondly, the spectating was awful. This is an issue I have in e-sports in general. Siege's competitive scene was a lot worse than OWL, but OWL still wasn't very good. Part of that most likely had to do with the lack of character variety and/or the fact that the game itself doesn't do a lot to make non-combat roles more interesting than "stand in the back and left click" but the camera would just be following around whichever Korean god was playing Widowmaker or Genji, and the match would essentially come down to them and even if everyone else on the team put the effort in to let that one guy do his thing, you'd never know it from watching the match. It's kinda hard to tell if that's more an issue with Overwatch in general or just OWL, but either way, it wasn't fun to watch. Bonus points for the dozen times a round that the camera would be following the wrong person during the action, and you couldn't see what was going on.
Thirdly, none of the teams actually felt like they represented the areas they claimed to. I suppose that's arguably an issue with all sports, given that sports, such as American Football, allow for trading players between teams. To be totally honest, I have never paid the slightest bit of attention to sports in general so I don't know how common that is and only know that because I have friends who won't shut up about it. But even so, sports teams will have a home stadium/court/field, and that's their base and it'll be located in their namesake. Conversely, Overwatch has a bunch of teams pretty much exclusively full of Koreans (wait, am I talking about Starcraft?) and yet they're trying to play up local pride? There's absolutely nothing tying your local team to the location in the name they were given. I would've preferred normal team names.
As a final note, I'd comment that I'm aware of how incredibly subjective all of this is. I'm thoroughly aware that none of these reasons are particularly rational, or even coherent. I struggled to figure out how to communicate some of my gut feelings here, but I hope the above is legible. I'd look for more specific, objective reasons for why OWL didn't work, but Blizzard really tried making it into a "real sport". I think they were trying to tap into the same type of emotional engagement that gets people so darn worked up over muscle-y dudes kicking a ball around. And at least personally, they absolutely failed at providing a game or an e-sport league that I connected with in that way.
