This I believe is not done for a variety of reasons :
1) Safety : A car typically has a gas tank in the boot area which is protected by the metal of the car. Accidents which are very likely to happen everyday like small dashes wont affect the cylinder directly.
In a bike the cylinder, if provided, would be very open and with the slightest slip or accident it might get damaged.
2) Weight : Because of the higher pressure under which CNG is filled (around 200 bar) and the safety standards the normal weight of a (10kg CNG) cylinder is 50 kg. Adding 50 kg cylinder to a normal 1000kg car is 5% weight addition but adding even a smaller 25 kg cylinder to a normal 100kg bike is 25% weight addition. This would affect handling, braking, etc in bikes. 5% weight addition in cars wont matter much.
3) Space: The dimensions of a 50kg CNG cylinder are length 835 mm and a diameter 316 mm which is too big to be fitted on a bike. Even if the smallest possible CNG cylinder is fitted the space required will be quite large. In cars, this kind of space is readily available in the boot. On bikes there is no such free space.
4) Engine power : Bikes, due to their compact nature, have engines that produce 8-10 bhp of power. Cars have engines that produce atleast 90 bhp. Now, addition of 5% kerb weight in a car wont matter much to a 90 bhp engine but addition of 25% kerb weight in a bike to a 8 bhp engine would under power the bike significantly, so it would be counterproductive.