Availability of fueling stations has been the big issue. Truckers will not convert to LNG unless there are fueling facilities, and it makes no sense to install fueling facilities if there are no customers.
UPS has done something about it, setting up a corridor between California and Las Vegas
The new LNG-powered tractors will pull trailers on a transit lane linking Ontario, California, and Las Vegas, Nevada, along with UPS's 11 existing LNG tractors.Installing LNG pumps at truck stops along I-95 every 200 miles would do a lot towards making LNG viable as a truck fuel, and would do a lot more for promoting energy independence than subsidizing the Chevy Volt.UPS plans to build publicly accessible LNG fuel stations in Las Vegas and will be able to access existing fuel stations in Ontario, California, and Salt Lake City, Utah, filling in an LNG trucking corridor that extends from California to Utah.
Clean Energy (T. Boone Pickens) is building over 100 LNG stations to put the claim on the that market.
http://www.cleanenergyfuels.com/pdf/CE-OS.ANGH.012412.pdf
The first phase includes 150 fueling stations with approximately 70 anticipated to be open in 33 states by the end of 2012 and the balance in 2013.