Why a bike cant drive in reverse dirn?

Becuase it does not have a reverse gear; reverse gear in a gearbox requires more cogs and drives so it makes the gear box alot more heavier than it needs to be. A bike is meant to be moveable by 'hands & feet'. In the early days (and in some military applications) some combinations (motorcycle & side car) did have a reverse gear; the old German BMW combo also had a drive shaft to the third wheel under the side car.