Why do you have to wait until level 20 for tier 3 runes?
Question is in the title. The only thing I can think of is that Riot doesn't want new players blowing all their IP on expensive runes when they don't have any clue what they're doing. I do understand the concept, but in reality, I have to question why tier 1 and 2 runes exist in the first place. If it's for the purpose of easing players into the game, why not give more of a tutorial on what exactly runes are, and how they're integrated into the game, rather than limiting every player (including those who are leveling a new account for a different position) to lower tier runes that, in the end, will be a waste of IP. Which, considering the cost of a full page of runes, new rune pages and champions (assuming you don't buy them with RP) isn't really something to laugh at for new accounts.