Probably because they produce them in such limited quantities and use white metal/resin for most of their kit parts. They're a small company so they can't afford the big polystyrene plants that Tamiya and others use, and they have fewer chances to make back money on the research and development of each kit. They counter this by making kits of race cars that other companies don't have and by releasing a crapload of aftermarket decals, wheels, photoetch, etc.