Postfixes are easier to use, if you know how to use them. They're not only used in programming but also in spoken language:
It is how, in this sentence, the verb works.
It isn't how the verb works, in this sentence.
Sentences that simple are equally understandable using the verb as a postfix or infix. When sentences get longer that changes, though:
It is how, in this sentence, which now has an extra clause, the verb works.
It isn't how the verb works, in this sentence, which now has an extra clause.
In the first sentence, it's clear that the extra clause is something the sentence has, not the verb. The second example has what is called a dangling modifier, which is a clause at the end of the sentence that ambiguously could be describing more than one thing, in this case it could mean that either the sentence has an extra clause or the verb has an extra clause.
The lack of ambiguity also means it takes fewer commands to describe a single operation. For example, in an calculator with infix notation, which is what most people learn, calculating the multiple of four pairs of sums would require the following keypresses:
( 5 + 6 ) x ( 2 + 7 ) x ( 3 + 8 ) x ( 7 + 9 ) =
With an RPN calculator, which has a postfix notation, it would only require these keypresses:
5 Enter 6 + 2 Enter 7 + 3 Enter 8 + 7 Enter 9 + x x x
That's 24 in the first example and 19 in the latter, a reduction of over 20%.
My grandma never really learned how to use a computer and was generally not very good at complex instructions. When she learned to use a calculator, RPN was just as common as postfix notation, and she was able to figure out how to use RPN, but had trouble figuring out postfix notation. Nowadays, schools usually only teach postfix notation, so most students aren't exposed to the easier method, and will have to do extra work, in the short term, to lean it, but it means calculations will require less work, once learned.
tl;dr: Postfix notation is the Dvorak keyboard to infix notation's Qwerty keyboard. The former is easier and faster, but everyone learns the latter. Putting in the extra work to learn the former will save you more time in the long run.