Propane has a much lower boiling point than butane. As such, it is impractical to keep pure propane under high enough pressures so it's liquid (cannisters would be too heavy and dangerous, etc). Since propane is more volatile, it performs better when it's cold. But since it's more volatile, the propane burns off first, and then you're left with the poorly-performing butane, and your stove is no longer hot. Cool, huh?