perhaps flight times are limited by the amount of gas on board.
I think that's the key problem. They need to keep the envelope warmer than the surrounding air to keep the craft aloft. And like us "stinkwing" pilots, they need a certain amount of fuel to control the rate of descent to the crash site - pardon me - landing point. They really only have single-axis (y) controls, and those feature terrific hysteresis, while the other two degrees of freedom (x and z) are brought to you courtesy of the prevailing meteorological environment.
Who would go up in one of those things, seriously?