Why is plane polarised light not the same as monochromatic light?

Monochromatic means basically "one colour" and it implies light of only one wavelength. Polarized has to do with the direction of the oscillating electric field.

Light of one colour, say red, is composed of waves all of the same wavelength but it can propagate with electric field components oscillating in every direction. So all the waves composing your beam of red light will have the electric fields oscillating in every possible direction. On the other hand polarized light has component (of all the waves) oscillating only in one direction.

You can polarize white light (that is the composition of many wavelengths=colours) and you'll have as output still white light but with electric field components oscillating only in one direction!