Question

why lpg is preferred than cng

Answer 1

LPG is heavier than air, which is less safe in a car crash than CNG. Nothing in your kitchen will ever hit a canister hard enough to rupture it, so a kitchen LPG fire is unlikely compared to a car crash causing a fire.  

LPG liquefies easily, so the pressure straight out of the cylinder is never very high. It's mostly liquid in the canister and only turns to vapor when released. That means you can use cheaper tanks and regulators -- only about 125 psi is seen at room temperature. 

In a car, the fuel system cost is less important than the safety. So LPG is less favored as a vehicle fuel.

hope it helps friend ♡♡

Answer 2

LPG most polluted in air and then CNG not polluted in air
CNG prices level low