I read somwhere that the extra cartridge length and higher working pressures of the WMR made it quite an engineering challenge to come up with a reliable automatic handgun for it that would be attractive in the market. The longer cartridge requires a deeper grip and longer receiver/slide, (meaning very little parts commonality with a 22LR line) while the higher pressures require stiffer springs. In a revolver you just stick in a different cylinder and you're done, but an auto would have to be a whole new gun - and one that could ONLY shoot the WMR cartridge. Given that the high ammunition cost would probably limit demand for the gun, the big makers (Ruger, Browning, etc) have just decided to take a pass on the project.
