When you have a full page of well-written dialogue that has a thought process to it, it is pretty easy to memorize. It's a lot easier to memorize than if you're in a scene and other people are talking, and you have maybe one word or one sentence that you have to interject at the right time and in a natural way.
One of the most important lessons that experience teaches is that, on the whole, success depends more upon character than upon either intellect or fortune.