They've gone through several renditions of punishment tiers, though as I recall, 2-weeks has always been the last straw before a permaban (though a long time ago, you could potentially get multiple 2-week bans before a permaban, without actually showing reform between them). What they found (and is still true today) is that most players correct themselves after their first warning. And most of the people who get hit by the system a second time will finally take it seriously and correct themselves then. But generally, if players made it through the system and were punished three or more times, reform rates plummeted to near-zero. That's why they escalate as they do now.
Also, regarding some of the other comments here... from what I've seen a vast majority of people who claim to have gotten a 2-week or even permaban as a first offense have actually later been proven to have gone through all the tiers just like everybody else. Also, something they don't seem to realize is that comments like "was banned for <specific game instance>" is also very misleading. Outside of very extreme behavior, it takes far more than one game of bad behavior to get any punishment at all. The system is built to punish for extended patterns of bad behavior across a number of games. But due to size limitations, Riot only sends 1 game's logs in the email notice, and up to 3 in the in-client ban notice. There are almost always even more games than the player is shown that triggered their punishment.