people are dumb and would think they could board in certain zones or that there zone was already called if they were calling some of the named zones. Moving everyone to numbered zones makes it a lot easier for gate agents to turn people around without issue and should reduce how many get in line at wrong time.

Delta would prefer to use your "Diamond" status as this post shows it makes you feel super special and builds brand loyalty. But there are enough flyers that don't pay attention or try and game system so the gate agents are wasting time turning people around to wait for the right zone. This might not fix it but it is a big enough issue that Delta is trying/hoping this fixes it.