One word. Infrastructure.
I have natural gas, so I could use it for local travel. The problem for me would be retro-fitting for on-board storage.
And a sentence fragment: Range and energy per unit of mass...
Can't natural gas be handled similarly to propane? There is plenty of propane delivery infrastructure in this country. Seems like it wouldn't be too difficult to augment propane with natural gas, without investing a whole lot in different technology.
Disclaimer: I don't know what the hell I'm talking about, okay? :)