[extraduder_ire]

I'd guess because it's less prone to get interfered with by wind, more easily controllable, and can stay up longer if battery power lasts overnight.

I too would like to see more autonomous hydrogen balloons flying. Even on tethers to get height cheaper than a tower.

[bloopernova]

I'm not well-read on the technology involved. Is it possible (feasible?) to use a high altitude balloon to get a plane to altitude?

You could either detach from the balloon, or use spare solar power to deflate the balloon in case you need it again. Maybe the balloon could help maintain altitude at night.

[woodlander87]

Perhaps not as high altitude as the stratosphere, but the US was experimenting with deployment and 'landing' of fixed wing aircraft from airships in the 1930s. https://en.wikipedia.org/wiki/USS_Akron

[hef19898]

They also tried building aircraft carriers out of ice in the 40s. Doesn't mean it was a good idea.