I've heard a couple of reasons - bus stops not correctly aligned so drivers fearful of being sued for people injuring themselves getting off buses at the rear doors - fear of people bunking onto the buses and not paying - the amount of time I saw our European brethren obediently waiting at the middle doors (where there was a sign saying exit by middle doors only) to be let off only to be shouted at by the driver to come up front!
I'd blame the drivers mostly, the buses were bought with double doors and then not used.
It's simple really - the middle doors could never be used as exits because that's where the fold-up buggies are stored!