because it's easy to play and accesible. like facebook games.
It's easy to pick up - that's hardly positive
It's free, and you can literally play it for free If you're content to play with like 7 champions
There's no huge commitments (no real leveling and gearing up) - that's just blatantly untrue. It has the stupidest leveling systems, and you literally can not get a decent match before you're level 30
It's actually pretty fun - if it's the only moba you've tried, maybe
It's simple when compared to HoN and Dota, which brings back the point of it being accessible - it's dumbed down, hence the retarded playerbase
Streams are vibrant and eventful - except they fucked up the largest event so bad they had to redo it the next weekend. players were cheating, afaik there was power outage at some point (or internet loss)
New champions every ~2 weeks, giving players something to look forward to - frequently adding heroes does nothing but make the game imbalanced. it ruined HoN and it probably has already ruined LoL
It's actually pretty fun - compared to other minigames of the same genre, like angry birds, farmville etc.
Easy grouping system so you're not wasting time - just like every other online game that exists
and last but not least, it's a pretty fun game. - well it beats watching grass grow
p.s. it's fun and a game should be fun