Cos they want(need) to release it at midnight. Enough people will be awake at midnight to login test the stability/possibly unnoticed game breaking bugs and (crash the servers) and give BLZ the time to hopefully tweek and fix such issues BEFORE the majority logs in ie. after school/work.
It make zero sense at all to have it released at any other time than midnight. World first pff, those charts can be adjusted to fulfill the need for e-peen fairness. All that matter is that BLZ get the maximum ammount of time to deal with the unforseen before the majority of players log in