I can't speak to LoL's quality because I haven't played it, but I personally believe the largest factor was LoL's business model. League was free to play and at

I can't speak to LoL's quality because I haven't played it, but I personally believe the largest factor was LoL's business model. League was free to play and attracted a huge player base because of it, while you had to spend ~30USD (?) to play HoN. They eventually switched to a free to play model as well, but it was way too late. I have spent thousands and thousands and thousands of hours in DotA, HoN and DotA 2—and HoN was a great game with a worse business model and sadly horrible leadership (CEO caught on several occasions dropping the most vulgar racist and homophobic remarks, banning people in-game if they upset him).