Why is Royal better vs Buttercream when piping?

On what? If on fondant, royal is better because it dries and won't smudge. It's also easier to pipe with royal then buttercream.

On buttercream, you can't pipe royal because it will melt. So you have to pipe with buttercream.

One is not necessarily better than the other. It all depends on the context of your design, as Jen mentioned.

I never, ever pipe with Royal. I just thin my bc so if flows out nice and smooth.

Ok, thanks! I have a related question. I did a cake dummy with white buttercream then piped it with blue buttercream, after 2 days the blue bled into the white and looked horrible. How do I fix that?

I have never had that problem, but I think meringue powder prevents bleeding to a degree.

You really can't 'fix' it.
Bleeding comes from using icing that has just freshly been colored; The darker the color the more chance of it bleeding; &/OR because the consistency of the b'cream is too soft.
Meringue powder won't help really. It can help the b'cream crust some but you have to use so much of it and then it makes the taste of the icing 'off'.