I understand why transverse waves can be polarised because their oscillations can be blocked by a polarizer. But, why can't longitudinal waves be polarised? Are there no polarizers, or something similar to that available for longitudinal waves?
With transverse waves, there is a choice in which direction (in which plane) the oscillations occur. For instance, let the transverse wave move in $z$-direction. Then the oscillations could be for instance in the $x-z$-plane, or they could be in the $y-z$-plane or they could be anywhere inbetween. In order to distinguish between these different waves (i.e. waves with oscillations in different directions), physicists introduce a parameter called "polarization" which describes the geometrical orientation of oscillations.
With longitudinal waves on the other hand, the oscillations always occur in only one direction, namely along the wave. There is no need to distinguish different oscillations direction, because there is only one oscillation direction. Therefore it does not make much sense to speak of "polarization" of longitudinal waves, because those waves are fully described by wavelength/frequency/velocity.