For the most part, its strength requirement.
Actually, Strength to weight, round tube is the strongest, if you compare equivalent cross section area. Thats why everything from race cars to baja buggys are framed up in round tubing.
That being said, round tube cross section will have a larger width than square, so if you are just comparing width, than square wins the strength, but it is not a fair fight.
The enormous pain or dealing with round tube is why everyone without access to a full machine shop uses square tube. Just think of joining two pieces of tube into a T shape. With square tube all you require is a chop saw. With round you need more specialized tools, and infinitely more production time.