The difference is PHEVs don't need recharging stations since they use conventional fuels for unlimited range. SWJJ is correct that
CNG refueling infrastructure doesn't exist in many areas which is why NGVs have primarily been used as fleet vehicles in the US. The home refueling options are expensive and have questionable durability and support.
Our gasoline prices are still too low for many consumers to justify the switch to CNG. After the $4,000 federal rebate a Civic GX and home refueling appliance costs $9,000 to $16,000 more (depending on refueling appliance) than a comparable gasoline Civic and still has a lower resale value. At today's prices an average driver would save around $250/year using service stations (
check prices here) or around $475/year with home refueling. Mass adoption will require lower NGV costs, larger federal incentives, higher federal gasoline taxes, and greatly expanded fueling infrastructure.