There's quite a long and detailed story behind the whole reason why, but since I'm on my phone I'll just give you the very short version.
There were a small amount of certain players who had a ton of low-level accounts, and those players annoyed everyone else with their trolling/shitposting. The second they got banned, they would get on their next level 5 account.
Eventually the level requirement was raised to level 7 across all boards, but that didn't stop the trolls in GD.
Tamat (a Rioter who doesn't work at Riot anymore) got fed up with this and raised the level requirement again to level 20 to post, and level 30 to create threads. This is only effective in GD however.
As to why the trolls didn't leave GD and begin to pester other boards that had the level 7 requirement, I have no idea.
The long version would probably be like 6 times the size of this post if I was to type it all out.