The trailer takes up space when not being used, the back half of the vehicle does not. This is true no matter what the parking situation is. Zero space is better than any space, if the back half of the vehicle doesn't need to be enclosed or have seating.
The trailer requires some extra maintenance too. Keeping things oiled, tires aired up and replaced regularly. Driving with the trailer means needing to use to parking stalls when parking. Backing up with trailers is a lot of fun too!
The trailer is a very situational benefit for situations where the main vehicle space is needed. Hell, if I need a trailer I can just rent one from UHaul. But I don't when the same thing is solved with a truck bed, and the truck bed format is far more convenient if the vehicle with the bed is the same size as a normal car!