It was a myriad of reasons, but the main ones were basically;

Removing the GC locks opened up the queue so filling a 72 player map became easier (even more so after they added a roulette with exp rewards making the mode pop frequently.)

8v8v8 created a constant premade group stomping so they clearly wanted to end that without hurting the ability to group up. Its clear with CC that they want to avoid premade problems considering in CC they don't even allow groups at all.