There are two reasons. One is that in North America it was only in 2008 that we started getting a tremendous supply of natural gas, and this supply has decreased the price. Europe is much more environmentally friendly. They do things like the 30% savings on carbon dioxide and 99% on particulates, the SOx and NOx, and so forth. There are 13.6 million natural gas vehicles in the world, but only 140,000 in North America.
On March 6th, 2012. See this statement in context.