So let's look at this logically:
STN: London catchment with lower in city car usage, predominantly short haul low cost thus less likely to have luggage:
London City: predominantly day trip business pax, need I say any more
LGW: Fair Enough, I'll concede that one but some element similar to STN of low cost less luggage and London city folk.
LPL: predominantly low cost again, with less baggage etc.
MAN: long haul with some flights such as Pakistan with 60kg luggage each and lots of family to see them off. Get them on a bus? No chance.
Charter pax with bags and families in tow. Again, get them on a bus trundling through the suburbs, or into Cheshire where public transport may as well not exist? Again, unlikely.
To compare MAN public transport uptake with the likes of STN/LCY and to an extent, LPL/LTN is just fallacy. It really is the cliche of apples and oranges, the traffic mix is by no means the same.