Thanks for all the laughs on this thread, takes me way back to the dark ages, well our first visit to the states, and my total shock of seeing horses in the pack of pick up trucks going down the highway, just a totally insane sight
This was someone who was already stunned at the sight of pick up trucks and gooseneck trailers, because although at the time you would see the occasional pick up truck, I had never ever seen a gooseneck until we visited the states.
The very idea of a trailer without a ramp was funny, I couldn't work out how you would get horses to load and unload with that step, all I was used to were bumper pull trailers and everyone had a ramp. Now I can't think why all the trailers in the UK have ramps, a step up is so much easier to deal with.
As to why this type of transport isn't seen so often, I would guess a lot of it has to do with trucks being built out of lighter materials, that will get damaged easier, we have a 1953 GMG 1 ton truck in the yard, I think I could put a horse in there and it could have a kicking fit and nothing would be even a little bent, the 2002, not nearly so tough. The older trailers were heavier and harder to pull as well.
Biggest reason though, I would imagine as others have said, when folk were using their horses for work, rather than leisure, things were more practical, and lets face it, farming and ranching has always been tough, and trailers for the horses would of been seen as an unnecessary luxury.