Tube frames are basically trusses anyway. You have two parallel cords and diagonal braces, just not very many of them. Once you shrink frame size vertically so that the unit can be made more compact you give up huge amounts of structural potential, your strength goes down by a the square, and stiffness by the cube. So if you go from a 20" separation to a single tube of 2 inches, the tube starts to want to be a little heavy, so a deeper truss made out of lighter elements starts to be attractive.

Tubing is pretty wonderful stuff and as nice as it is, the only reason it is cheap is because so much of it gets used for so many diifferent applications, that it ends up reasonably priced.