You need a lot of interior baffling or insulating material stuffed inside an air mattress to get the R value up to 3 or 4. In a big open tube you get convection, which speeds up heat transfer.

Note that the original NeoAir, which is about the same thickness, is only R 2.5, but that's with a lot more interior baffles plus reflective coating on the inside. The exped mats are filled with down or synthetic insulation.