In transverse waves the propagation is perpendicular to oscillations. But in longitudinal waves the "oscillations" are actually areas of more or less rarified medium. They are along the propagation of the wave. To polarize waves is to filter out wavelengths of certain orientations. This could be possible for a transverse wave because the oscillations can be oriented in any way away from the vertical or horizontal axis and remain perpendicular to the motion. But longitudinal waves do not even have oscillations that can have an orienation.