Good question. After all, a common backyard grill or a two-burner Coleman camp stove uses propane and works in shivering conditions. But pick up the empty fuel can and it's heavy. The answer to your question is simple. Propane has a high vapor pressure under normal conditions (that's why it works so well) and therefore requires a very strong and heavy container. The can itself is heavier than you would want to carry in your backpack. Going to the next lower vapor pressure simple hydrocarbon, butane, reduces the strength and weight requirement for the can. The problem there is that, as discussed by Hikin Jim, the vapor pressure of butane gets too low to be useful as the temperature gets cold. The butane isomer, isobutane, has a slightly higher vapor pressure and adding a little propane (20% or so) makes a mixture that is fine-tuned for the vapor pressure/strength-requirements. So we end up with something of a compromise: a fuel that has a vapor pressure that is just high enough to be useful under most backpacking conditions, in a can that is not too heavy to carry.
