Outrageously expensive labor. You need a union electrician to run a power cord, a union decorator to put your banner on the table, a union teamster to move your gear from the dock to the table, etc.
My (least) favorite trade show experience was in DeToilet. One union person to carry a table, a different one to put a tablecloth on said table, and a third to staple a drape onto the table. This was during the time that FedEx could not drop off packages at the Convention center loading dock because they were a non-union shop.