That really bugs me. It seems to happen with increasing frequency.I think The Americans was being shown on 5USA when they stopped showing it halfway through.
Channel 5 have a habit of doing that on their various channels. They didn't show the final season of Sons of Anarchy nor the final season of Justified.
I assume that the producers ask for too much money for subsequent series, not realising that the show became popular because it was shown on minor terrestrial channels and gained a following.
Or, having gained a following, it was picked up by pay2view channels.
Either way it’s bloody annoying.