There is a reason for that. The EPA and CARB want every kit certified for every possible combination. Each test comes at considerable expense. Its all fairly wasteful considering that CNG drops emissions across the board on every application. Manufacturers and resellers of kits must concentrate on high payback scenarios- these are fleets and .guv vehicles, leaving the average consumer out of the game unless you want to buy a dedicated NGV. Luckily our state lawmakers saw the folly in this and passed a bill that allows conversions to be done by CSA certified techs. These then have to pass the same emissions standard as the year they were built. Seems far too sensible, doesn't it??
My dad has been driving a dual fueled CNG F-350 for years. His came from a gas company auction. He claims that it is cheaper to drive per mile than his motorcycle. Makes sense when fuel is 4 times the price- even a truck that gets 10 miles to the gallon will cost the same to run as a car getting 40 mpg. Now that fuel is coming down its closer to 3 times.
The company he worked for (retired) has a fleet of vans that run on CNG. 6 of them have gone over 1 million miles. Now that's Ford tough!!