You can always move more air pulling it then you can pushing it. Seal it up good and create a vaccum situation on the backside of the radiator, the air has no way to do anything other than move through the radiator.
With a pusher, the air will bang into the radiator and rather than flow through can impede all flow all together. Obviously some gets through the rad or they wouldn't work at all. Just not as good.
Like trying to push a wet noodle or pull one. You'll have alot better luck pulling it.