Once upon a time, when exposing my knees in public was considered only normal (because I was a child and Long Trousers were grown-up), there used to be people called conductors (and, yes, conductresses) on buses.
And one of the things they did was to chivvy people upstairs to a seat, rather than let them block the lower deck and gangway.
Nowadays, of course, drivers have got their hands full driving. On some models of bus, I rather think, they have no way of knowing whether or not there are seats empty upstairs, even if they felt inclined to get into managing passenger behaviour, which understandably they don't.
For those of us who manage to get upstairs, it makes for a rather pleasantly uncrowded ride (provided there aren't too many teenagers who treat it like party central). But why do people herd together downstairs like that?
I'm not grumpy (much), just puzzled.