Why reverse polish notation is bad
In one word: It's unreadable for us humans. Just look at this example in RPN:
1 2 3 4 5 6 + - - + -
How long do you need to calculate the result if you just read along letter by letter? A long time, because until you reach the first plus sign, you simply can't do anything at all but remember the numbers. And we're bad at remembering many things.
Just to make this clear, do the same in in-fix notation:
6 + 5 - 4 - 3 + 2 - 1
Reading along letter by letter, when you get to the last "1", you already have the answer of the calculation. This is not because we are used to infix notation more than RPN - it is just because it is the natural way things work.
This also is not only the case for mathematical formulas, but also language. If I say "I gave you a book.", then in RPN it would be something like "I you book a gave". Or "I went to the shop and bought a book." would be "Book a I shop the to went bought and."