They all had different reasons of course. Some didn’t want to invest the time to climb to GM. Most of them hated the Abusive Chat report system. It tended to punish them more as they played almost everyday.
There was also the slow down of content after HOTS going into maintenance mode in late 2018. I think too, just a certain level of boredom, most people lose interest in games eventually. Many had been here since alpha/beta so it’s understandable.