I think the answer is something about the way of hunt. Usually, American hunters likes to hunt one of each specie and they spend a lot of time with this animal. For example, is common to hunt "only" a white tail in a year.

Europeans usually prefer to hunt more animal in one year, we've some "don't-know-why" preferences (boars, buffalos, roe deers) and hunter use to take several of them each years.
About me, I've hunted more than 100 roe deers and I'm only 33 years old. I think there are no trophie room enough big to store all those trophies, if shoulder mounts in a future, but I can put all of them together in skull mounts.
Anyway almost my african and american trophies are full or shoulder mounted.