Thanks for the explanation. I don't agree with your point that postfix is easier; maybe it's because most people are not used to thinking in postfix. It can be faster and easier for computers, but we build software for people, and it increases cognitive load on the users.
It might be fewer keystrokes, but I feel for most people, the difference in keystrokes doesn't justify the cognitive load it adds.
I grew up using HP RPN calculators and it was actually easier in the long run. It isn't in the keystrokes either, but because when I was calculating an algebraic expression, I would have to setup a "query plan" in my head on how I would input all the values and then often I could simplify things as I entered them. I got rather good at manipulating algebraic expressions as a result, and it served me well in college.
It might be fewer keystrokes, but I feel for most people, the difference in keystrokes doesn't justify the cognitive load it adds.