It was causing server issues, not just in Clash. The issue was if we proceeded with the launch even normal games would be lag fests and multiple drops would occur. Rather then potentially crashing the game as a whole... a call was made.
I warned people four hours ago it wouldn't happen tonight. Riot was even advised 6 hours ago to give proper warning, but decided to be stubborn and see if they could fix in time. Be glad you got more warning then Europe, they were told 25 minutes before launch.
Honestly, they should of cancelled the event this morning when the problem occurred but they were determined to not have a colossal PR nightmare on their hands after advertising the event on youtube/gaming sites for weeks. Waiting just did more damage.