This is way overdue. The CH Beis Din seems to have a delay in their guidance.
Was there ever any substantiated expert guidance advising that outdoor gatherings were not a problem?
Well, in EY when they announced the 10 person limit they said to maintain 2 meter distance. I think totally banning minyanim is an extreme measure taken due to the severity of the situation on one hand, and the fact that it was not being taken seriously enough on the other hand.