Having built equipment trailers, on and off for over 40 years, the wide space is used to spread the load. It is more inconvenient to spread them because you aren't using a standard equalizer. Semi Truck trailer manufacuters do this for the same reason. Spreading the load does make a difference. You will notice it being common on alumimum frame Semi Truck trailers.
For a TT it is cost saving for a frame that isn't as strong as one required with a standard spacing axle setup.