Riot Accounts

For my PBE account (the one I'm currently using) I'm being asked to update my account and choose a username. It says the username I'm trying to choose is already in use, of which I'm assuming is being used by my live account. Why can't I just combine my live account with my PBE account and spare myself the trouble of coming up with a new username?
Report as:
Offensive Spam Harassment Incorrect Board