Multiple axle trailers have several advantages over a single axle. With more tires/wheels, you will have an increased sidewall stiffness and load capacity, double the amount of brakes, a smoother ride, and a higher security if a blow out occurs.

When a single axle trailer hits a hole or drops off the edge of the road, it will jolt the trailer rather badly. With two axles, the second one will temporarily support the load, smoothing the trailer's passage.

Commercial aircraft have been using multiple wheel assemblies for many years now. At one time they used large single wheels of sufficient load capacity to carry the heavy weights. Now they use multiple boggie systems and smaller tires for the same loading weights. These increase the stability of the landing gear, reduce the weights of the individual tire/wheel assemblies, increase the amount of brakes and their swept areas, and reduce damage to the equippment if there is a blow out. These are all common features you want in a trailer as well.

Another consideration is the resale value. Single axle trailers do not hold their value well and are difficult to resell. I have looked at a couple over the years that were in a like new condition. The owners had to continually drop the prices over lengthy times, in an effort to sell them. One owner finally gave up and donated his to a shelter group, using it as a write off.

I own several trailers of a single axle variety, boat, utility etc. Hauling equipment is quite different than livestock. When an animal is concerned, its safety is paramount. When expensive trailers are concerned, I don't want them damaged if and when a blowout happens. My horse trailers will always have multiple axles.