Well, the buoyancy isn’t the only reason they went with hydrogen. It’s also a lot easier to obtain hydrogen gas, as it is a lot more common on planet earth than helium. Helium is a noble gas, so it doesn’t combine readily with other elements, and being lighter than air, it is constantly escaping to space.
Hydrogen, on the other hand, is a very reactive element, so it is locked up in heavier molecules and doesn’t escape. So we have literal oceans of hydrogen available to us, versus the limited and dwindling supply of helium.
Yeah I was thinking about that afterwards. We can MAKE hydrogen from water with very little effort.