HoN is just suffering a slow death. It will die eventually since DoTA 2 and LoL have a lot of competitions with good prize money and sponsors. Also a lot of big pro gaming organizations have teams in one or both of those MOBAs, so any smart pro player wanting to get into the genre will go to either of those two games. While HoN may still have a niche following, at the end of the day, it's community is dwarfed by both DoTA 2 and LoL. One only need look at the reddits for the three MOBAs and look at the number of subscribers. LoL has 455,000 subs, DoTA 2 has 136,000 and HoN only has a measly 10,000.
But to answer the OP, when HoN came out, it was pay-2-play, you had to buy the game, so the community was of course smaller. With LoL it was F2P from the get go. Being F2P meant a larger community and therefore greater interest. This of course translated to more people streaming LoL, more people watching and thus games conventions were more likely to host a LoL event than a HoN one. Just look at http://www.twitch.tv/directory and see which are the top streamed games and where HoN is on the list.
Even in release timing, Riot had S2 beat. LoL was officially released in Oct 2009 while HoN was in beta from April 2009 to May 2010, when was officially released. So one can see that for people new to the MOBA genre, if they had to pick between the two at the time, one was pay-2-play, another F2P, they'd likely try out LoL first and possibly stick with it rather than fork out cash for the other.
Then came DoTA 2(also F2P) which had the added bonus of being made by Valve and the addition to being the sequel to the MOBA that started it all. Valve also organized a massive DoTA 2 tournament in August 2011 to launch the game with a whopping $1.6 million prize pool. HoN had just gone F2P in July of that year, but it was already too late.
