There is no explaination for this, but English speakers ride on public conveyances (busses, trains and planes) and in private ones (cars, trucks, vans, motor homes and taxicabs). Of course, for obvious reasons, people ride on motorcycles, horses and scooters.
I think that the reason is similar to that for motorcycles-- in the case of a larger vehicle, the impression is of being on a broad surface rather than inside a restricted container. Hence we travel on ships, planes and trains, but in taxis, and either in or on buses.
It also depends on context. 'We went to St. Louis on a Greyhound bus. When we stopped for dinner at Mobile, I thought I had lost my wallet, but I had just left it in the bus.'