Well, it's mostly because the story in GT was overall terrible xD So fans decided to either forget it existed or not consider it anything to do with Z.
By today's standards (SSJ God and so on), Nappa was a flea lol But at the time, he was a monster. Considering power levels at the time, I reckon most people could have destroyed a planet if they wanted to lol
Anyway... back to Bleach!
It's a GUY?! I thought it was a girl... lol oops.
I'd say we need more info on what training they've had up there, tbh. We only ever saw Ichigo and Renji doing anything, iirc. And Byakuya is still there as far as we know.