Why (in-universe) can you only scoop from main-sequence stars?

I know that the stars you can fuel-scoop from are the main-sequence ones, but why are these the only ones you can scoop from? The obvious idea would be that they are the only ones with free hydrogen, but (an astronomer friend tells me) T-class stars still have hydrogen, and there's some controversy over whether they should really be considered main-sequence.

Is there some in-universe explanation (official, or even something informal like a developer interview) of the mechanism of the fuel scoop, and what's special about OBAFGKM-class stars that allows the scoop to work on those alone?

The drives in Elite: Dangerous are supposed to run on hydrogen fuel (at least in Frontier and FFE).

Main sequence stars (OBAFGKM) during their main sequence phase contain an abundance of hydrogen (approx 75%) which can be scooped up and used as fuel, typically once this is used up (to some extent) stars move off the sequence. The other star types do not have this abundance of hydrogen and therefore would be less suitable or even completely impossible to refuel from.