Well, they’re not a permanent dwelling, and they lose value as they age – as opposed to a house, which gains equity. For this reason, older trailers were cheaper to buy and to rent – perfect for low-wage earners.
Additionally, the older, smaller (real) trailers were small enough to be hooked up to the back of a vehicle and toted around. That made them good for people who were transient in nature. I don’t believe that there has ever been a society who’s looked kindly upon nomadic types.