About 85-90% of people are right handed and that’s a good enough majority to build a product.
It's sad no one bothered to copy ideas from Windows Phone 8, its UI had a lot of thought put into it and was exactly as efficient to use for left-handed users.
To support a left handed person it should fold open to the right, also that way it’s a bit weird, although I’m curious to see how the animations would work for notes apps for Arabic language UIs where text is written from right to left.