Hydrogen offers low drag and good thermal conductivity: http://en.wikipedia.org/wiki/Hydrogen-cooled_turbo_generator

Why wouldn't they spin instead in a near-vacuum, instead of an environment of hydrogen?

Because near vacuum is a terrible heat conductor.

Vacuum has no drag but also no thermal conductivity, so you'd have to cool the generators some other way.

If they're sealed and there's no oxygen in there, then there's no boom.

Since hydrogen is so light it makes for less aerodynamic drag on the moving parts.

Ah ok makes sense, thanks!