Pictures of medieval encampments show mostly round tents, with the occasional large double bell and a few more rectangular tents. See?
Lots of people want a tent that they can fit a double bed in. But if you do that with a circular tent you can end up with quite a large round tent.
It seems that the goal of historical reenactment conflicts with the desires of the modern world.
Edit 28/1/13: Actually, I’ve just designed a round tent that you can fit a double bed into and isn’t too big. when it’s packed it’s smaller than a modern tent with the same floor area is likely to be, because it only has one pole, and it’s only 4.2m diameter and 3.5m high. The trick was remembering that I designed the circus tent to fit the frame for a double bed sized set of bed curtains between the pole and the wall about 1.8m in the air. When the double bed is lower (in the most recent case, 0.7m) then the total bell size and height can be smaller.
I’m going to keep looking for encampment pictures. If you find any can you add them in the comments?
added 14/1/2012: Landsknecht encampment woodcuts