I think it has to do with rather low barrier for entry and the ever elusive 18-35 demographic that advertisers keep chasing. If you have good enough ideas, they can market almost any product. Liquid as a brand also has a pretty dedicated following and they managed to diversify into multiple games, which keeps the risks low and they keep very clean and likeable image (mostly). Thanks to the community support, the watchers might even be inclined to use less adblockers. So if you want to enter the field, there aren't many other brands with the same resume. LoL teams are for the most part focused on LoL, otherwise what is out there ? Fnatic, Dignitas that just got investors too, CIS teams that aren't very famous in NA and, like, CoL ?
It's not quite like the chinese buying Dota 2 teams, but I don't think they expect huge returns in the next 5 years, however the horse is here to stay and for how long has Nazgul been working on the whole thing, I think he deserves the payday.