Here's a theory for you:
Political Parties want you voting emotionally rather than intellectually.
If you vote with your emotions, you're easier to control. Suppose you're with a group of people and you're trying to sway them to your side. The easiest way to do this is to convince them you have a common enemy. Having only two groups to chose from makes this easier. There's no thinking involved, merely a choice. This is why we see so many 'Hate' ads. They work. They control you.
If you vote with your brain, you're thinking about the issues and you're holding the politician responsible for their actions. You're no longer controllable. Now, you have the power.
It's a theory. I should apply for a government grant to expand on it.