Perma

Because Riot had tried what amounted to a permanent chat ban before - more specifically, an escalating Chat Restriction system that stacked indefinitely. People under longer-duration Chat Restrictions simply resorted to trolling their teammates as a way to express their toxicity, so, Riot decided to cut out the middle-man and not give players who make repeated chat-related offenses the opportunity to turn to a much harder to track method of releasing their toxicity.

In other words, permanent chat bans just don't work. They don't really stop toxic players from being toxic - they only narrow down the ways they can be toxic.

This is a question we get a LOT. The simple answer is that Riot tried it with the first two rounds of chat restrictions and found that people who couldn't be toxic in chat found other ways to grief the other players. There wasn't an effective reduction in toxicity.

Because that doesn't stop players from running it down lane, afking after 3 minutes, or role stealing.

So Riot can get more $.