There were three H 1's that also participated in the Nevada Trophy they year we did. I say participated and not competed, because they all were on bigger tires than the competition allowed. I think the smallest tire size in their group were 37's. With that being said, they could have been allowed to compete, because their width prevented them from being contenders.
At one point in the competition, there was a long, tight canyon with a technical waterfall. This route contained a bunch of high point value waypoints (That should have been an indicator to it's difficulty). Depending on your route, you either had to go up the wet waterfall, or go down it. You had no way of pre-planning this or knowing that there was a waterfall due to the way the waypoints are distributed. The route our team navigator selected had us dropping down the water fall. This feature stopped the H 1's. They could not crawl up it, nor were there any suitable anchor points for them to winch up it. One of the H 1 guys joked about not wanting to up root a large juniper to get to the top.
Keep in mind, this is in wide open Nevada and their width was enough of a factor to prevent them from being competitive in a field that consisted mostly of Land Rovers with a few Toyota's.
For the sheer visual impact, it's really hard to beat watching a bunch of properly kitted Land Rover's bomb across the desert. With that being said, The H 1's looked awesome flying across the Nevada desert three deep.
Is this the extreme end of the spectrum? Yes, it is. Do most "Overland" trips consist of competition and rock crawling? No, they don't, not even close. It's a very different mentality when you're driving with your home.
As far as why don't you see more of them outside north America? I'd say there are a lot of reasons, some great ones have been mentioned already. The basics boil down to security and fit. I've considered a wagon from time to time, but I would not travel outside of North America with it.
Gear (Justin) had one, I think he sold it for Jeeps.