Based on what I read in
this thread, Encore Vehicles built new bodies for the M151A2 and then used Military Surplus components for the suspension, steering, powertrain, etc.
Apparently due to safety/roll-over concerns the U.S. Government would not sell an operational M151A2 at a surplus auction - they would destroy them by cutting the body in half. Accordingly, there are a lot of parts for the M151A2 but not many bodies. Encore Vehicles developed a business plan where they'd manufacture the bodies and then use surplus parts to finish building the vehicle.
I have no idea why Encore Vehicles went out of business - maybe there's not too much demand for this type of vehicle when there are so many other attractive options.