'unfamiliar' is 'unintuitive'. You only have an intuition for things that are similar to things you have encountered. Some unfamiliar or unintuitive things might be simpler than familiar things. But, at first, it is often easier to use familiar things, even if they are less simple.
Regarding RPN, I am not convinced that they are actually simpler to a human. In order to understand an RPN expression I try to keep the stack of operands and intermediate results in my ultra short term memory. This can easily exceeds its typical capacity, whereas when trying to understand an infix expression, I can replace subtrees with an abstraction, i.e. a short name, in my mind. But perhaps I have just not yet seen the light and spend enough time working with RPN expressions.
On the other hand, RPN expressions are certainly far simpler to implement.
Regarding RPN, I am not convinced that they are actually simpler to a human. In order to understand an RPN expression I try to keep the stack of operands and intermediate results in my ultra short term memory. This can easily exceeds its typical capacity, whereas when trying to understand an infix expression, I can replace subtrees with an abstraction, i.e. a short name, in my mind. But perhaps I have just not yet seen the light and spend enough time working with RPN expressions.
On the other hand, RPN expressions are certainly far simpler to implement.