If you want a reason why Rioters are more active on Reddit than on their own boards.
Just check the upvoted threads. They are 80% Riot bashing and people venting on recent loses. Why would they take time to get used to the interface while the Reddit conversation is way healthier anyway.
The boards honnestly look like a bunch of butthurts that needs to vent. (This is highly insulting, but I feel the need to take the same tone as everyone else)