To explain it a bit better if you could see your opponents cards two things would happen when your drawing:
1) Youd see your opponents cards dont match any of the cards you need and therefore the chances of you getting your draw increase.
2) You see your opponents cards and they are cards you need to hit your draw and therefore your chances decrease.

But because you cant see your cards these two cancel each other out and therefore become useless.

Sorry if that made it more confusing.