Why cant bikes use CNG in place of petrol?

They could but it would require a lot of modification. One might also ask, what would be the benefit from using CNG on motorbikes. The real reason for using CNG on engines is for saving fuel. As most motorbikes use far less fuel than cars, and are already more economical than cars, changing them to use CNG seems kind of purposeless. Another problem is, the CNG tanks have to fulfill far higher specifications than normal motorcycle gas tanks. This would increase the overall price of the motorbike, besides the tanks would look ugly in comparison to the normal gas tanks we have on motorcycles now.