Why is the onion root tip used so much for showing mitosis?

It's funny. We just did the onion root tip lab in my biology 1 cl (I'm in 10th grade) where we had to identify prophase, metaphase, anaphase, etc, and that night I went to see Twilight and they did the same exact lab! So I was just wondering why exactly the onion root tip is used for showing mitosis in plant cells so often.