Because that would empower minor candidates. The parties that are in power aren't going to change the system in a way that disadvantages themselves and perhaps

Because that would empower minor candidates. The parties that are in power aren't going to change the system in a way that disadvantages themselves and perhaps even poses an existential risk. That means if you vote democrat or republican, you're voting against your proposal. Neither party is so kind-hearted that they'll cut off the branch they're sitting on!