It's because when upright the force of the bumble bees colliding with each side is in equilibrium. As the bike starts to lean, more bees collide with the upward side. As the bees can't be there (because they're incapable of flight) this creates a pocket of low pressure, so air pressure pushes the bike upright.As the kinetic energy of an impossible bee is proportional to the square of the relative velocity, the stabilising effect increases with the speed of the bicycle. At rest, the non-bees are too small to keep the bike upright.Consequently, a bike cannot self-stabilise in a vacuum, as there would be no air pressure difference, and all the bees would asphyxiate.Time for more codeine...
