I think there are a number of reasons that this happened.
One, LoL does a very thorough job of promoting its pro game. The game's UI let you know when a company event is upcoming. It was easy for a curious player, like me, to get hooked.
Second, playing during the spring and summer are times that are normally sports-dead for me. I don't consistently follow hockey, and I don't care about basketball or baseball.
Most e-sports games are available for free on the internet. Sure there are a few commercials, but nothing like those on TV. Also, I haven't really been living in places that had a TV or had one hooked up for watching television.
But the big one for me, especially vis-a-vis football, is that watching e-sports doesn't trigger guilt in the same way. While I can't say for certain that e-sports athletes aren't financially exploited - I have no idea how much they're actually paid versus how much revenue they generate - I do know that they aren't at risk of concussions, ACL tears, or other major injuries that can not only shorten their careers but also have major health complications later in life. As much as I enjoy following football, I have to acknowledge that it's a blood sport, and nothing we try to do will change that. With e-sports, not having to put that asterisk on my enjoyment apparently really matters to me.