There are probably lots of reasons, 55 seems a little on the slow side to me.
However, there are a number of reasons/rationals, none of which have anything to do with "we are taking away your rights".
1) Many trailers had, and probably still have, tires rated at 65, so excessive speed is not good.
2) Trailers (less so for 5vers) can start to sway uncontrollably when people drive too fast. Unfortunately, many people view a speed limit as the minimum speed, even when pulling a trailer. So, if the speed limit is 80, they will load up the truck and trailer and lock on the cruise at 85+. I experienced this first hand driving through Utah, where they even allow double pull trailers. Talk about a white knuckle ride.
3) Trucks with trailers take further to stop. You don't really think that a 15000lb 5ver on the back of an F250 stops as fast as a passenger car do you? The faster you drive the more time to stop, the heavier you are the more time to stop.