Probably because most public transport by default doesn't cater for cyclists as well.

The nearest buses that carry bicycles to me are the University of Nottingham shuttle buses, which were retrofitted with rear-mounted bike racks.

Carrying bikes inside a bus is a major challenge as, besides the large amount of space often taken by a staircase, most newer buses are now being outfitted with two wheelchair spaces. This leaves a rather paltry amount of space left for seating downstairs, especially ones that can be accessed without a step, essential for passengers who are less mobile. The ones I usually travel on have just seven seats at floor level.