Why? Because there are just too many RP sellers, Elo boost spammers, and scammers. Riot games have said that private chat and chat rooms will both still function, but public chat rooms will be shut down until they can develop the tools to help moderate them:
"As an immediate action we’re going to disable the public chat rooms until they’re useful and accessible. In their current shape they just don’t work and can actively create negative experiences for many players (especially new players).
We’ll keep all private chat rooms and messaging available to everyone, and bring the official public chat rooms back as soon as we can."