Trailers sit higher than they used to because of slides. Older non-slide trailers had wheel wells that protruded into the interior space. These were usually hidden under the dinette or a couch or something. Trailers with slides have to have the floor higher than the tires so the slide will go out; thus the higher trailers.
As for trucks, I agree, they keep jacking them up to appeal to young people who think it's cool I guess. For someone like me who uses a truck for work the higher bed and sides make it harder to load things in and out and harder to reach my tool bins and see into them. I don't know if it will ever change.