> 
> Why is this conversion not bigger in the US? (aside from availability of
> propane)

Up till now, gas has been too cheap for people to be interested. LPG also 
has the minor disadvantage of filling stations being few, plus you can't 
just grab a gas can and get a gallon or 2 when you run out on the side of 
the road. It's actually used quite a bit for forklifts and fleet vehicles. 
Lumber yard I used to work for had eight trucks that ran on LPG.