It comes down to whether or not a POCO wishes to mount unit(s) over certain weight on certain sized poles and/or wish to do additional construction to handle larger sized units, like a platform on multiple poles or something of that nature vs using a pad mount unit or even using open units within some enclosure at grade level.
Rural POCO here typically will not hang over 3 50 kVA units on a pole for three phase banks, or over a single 100 kVA unit on a pole for a single phase supply. Have seen some three phase in single units, but most are older and get replaced with a bank of three units if needing replaced