The obvious response is that they're politically biased to the point where they'd sacrifice the story for the message. You see the same thing with Christian movies.
But we have plenty of inspired works that fulfill a political role i.e 1984, Brave new world, etc. Those stories have a clear message, but they are regarded as masterpieces and I'm inclined to agree.
So what is it about leftist propaganda that tends to make them absolutely terrible a good majority of the time?