Sounds like Mario has been thinking about this as well. I wrote out my answer and didn't see that he'd replied already. As you can see my reasoning is slightly different from Mario's, but that is why we make a good team.
Bearing design; is a field of it’s own, bearings have to be designed to withstand thrust load, radial load, and shock load.
Vehicles wheel bearings are either selected or designed for the specific vehicle and its intended use. So there is a high degree of customization of the wheel bearings for individual vehicles.
Trailer bearings are general generic, selection is determined by the weight of the trailer.
Lubrication; Modern vehicle wheel bearings can be pre lubricated and permanently sealed, or the bearings are in an oil bath, or the bearings are splash lubricated.
Trailer bearings are lubricated by grease, the bearings are either hand packed or packed with a bearing greaser. (There are a few manufactures who make sealed oil bath bearings for trailers).
Construction: Bearings can be made from ball bearings or rollers. Ball bearings have less contact surface area allowing free movement and lower friction. Rollers have higher contact surface area, produce more friction and heat. Ball construction can transmit destructive shock load to the race due to the lower surface area and high pressure. Rollers distribute the shock load over a larger area and the shock force is reduced.
Vehicles tend to use ball bearings, trailers bearings are almost always rollers. Rollers are great for the constant shock loads that off-road trailers experience, but not so great for high speed road travel where they create friction and heat.
Quality: This is based on personal experience rather than anything empirical. I have seen trailer bearings fail after 500 miles, while others will be good for 100,000 or more. Trailer bearings are generally very inexpensive, and in my opinion the reason they are so inexpensive is that the material they are made of is questionable and the quality control is low or non existent. AT uses high quality Timken bearings for this very reason.
Installation: My experience with vehicle bearings is that the installation instructions are very specific and the bearings are pre loaded to a certain torque setting. Most bearing installations are done by trained technicians.
Bearing installation on trailers involve a certain amount of art as well as science. The pre load is done by feel rather than torque setting, and it is usually the owner who does the installation.