Interesting. Why does the donut chart work then? They are somewhat similar.
Discussion
It's the area of the triangle that is hard to grasp for us. With the donut it's more like a bar. Focus shifts to the circumference of the circle instead of the area. Relative parts or the circumference of the circle do work good with our brains.
Bars are almost always better btw. But if you really want to use a pie shape, use a donut.