Here is the thing: While they will visit sometimes, the reality is Reddit is more predictable and more censored. With Reddit, there isn't really any chance of things derailing as anyone who goes off topic or says something deemed unpopular is removed. This means overall discussions are, for all real intents and purposes, scripted, as the only ones allowed to be popular in the discussion are pushing the idea Riot wanted.
What this means is, sure the boards may be better for the users, but from a business perspective, Reddit is superior. Businesses don't like variables, they like reliability. If Riot wants to push an idea, they don't want to risk it backfiring so they will simply go to Reddit. Furthermore, this makes Reddit less stressful for individual Rioters, as unlike on the boards you are not allowed to harass them.
Ideally, an environment could be made where people could disagree with Rioters as strongly as they want, but couldn't just outright insult them (as they are still people even if they represent a company). However, as of right now no such environment exists. So Rioters need to choose between a safe, organized, and scripted environment with Reddit, or an individualistic but chaotic environment with the boards.
