Because the mobile plasma charges(i.e. electrons first) will rearrange themselves because of the field, so as to resist it. But if the field changes sign before the electrons can move, they are 'confused', i.e. they start moving one way to resist the field, but the field sign changes sign before they are in position to repel it; on the contrary their new positions now aid the field; so the electrons again try to rearrange themselves, but the field has changed again. So no way to fight it. This is in very simple language what happens.