I’m not that old, but even in my lifetime trailers have changed quite a bit.
I still remember when a 7ft trailer was considered “extra tall” or “thoroughbred height.”
I remember when many straight load trailers were ridiculously narrow with flimsy fixed posts and chains for dividers.
I remember when there were no windows or escape doors.
Padding was non-existent, or just thin naugahyde awkwardly placed on the walls.
I remember when plywood dividers to the floor were the norm.
Everything was steel and frequently painted a dark color (hides the rust better).
Mangers were common.
Ramps were rare, or they seemed like they weighed a metric ton.
Then slant loads came on the market. Around the same time, we also saw the rise of aluminum and fiberglass construction. And improved suspension, improved matting, improved padding, improved ventilation, etc.
I think all of that, combined with data that horses often stand on a slant, led many to believe slant loads are superior.
But the thing is, many newly manufactured straight loads are nothing like their counterparts from the past. Tall, wide, sturdy yet moveable and well-padded dividers, no fixed posts, large windows and escape doors, user-friendly ramps, etc.
If you try to load my horses into the average 70s/80s 2H bumper pull, they are probably going to lose their marbles after a few trips. But they practically beg to hop on a modern, warmblood-sized, cushy straight load with all the bells and whistles.
But bottom line, I think people’s preferences are solidified by their early experiences-- both positive and negative.