Found this on the interweb when I was wondering the same thing some time back:
"Trailer tires have thicker construction for bruise resistance and more plys in the sidewalls for sway resistance and load carrying ability. They can be used as a vehicle tire but the ride will be terrible and kinda twitchy on the highway, they are really meant to be a follower tire with high sway resistance
The radial trailer tires are basically heavy bias plys with a thick radial cap for cooler running on the highway. If you run them on a regular vehicle it will ride like a semi truck.
In some cases like farm use their durability might be preferable, however as was mentioned they are not designed for high speed use (over 60mph) and get real hot if you run them at high speed. The thick tread keeps the heat in and causes them to delaminate at high speed.
That's why they have high pressure ratings, this lets them run cooler. If you run them at low pressures (30psi etc.) they self destruct due to the sidewall flexing which delaminates the cords from the rubber."