The point is why people have more trouble parsing (+ 2 3) than they have parsing (2 + 3).

I agree with you (contra GP) that path dependency is a fine reason to prefer infix. Continuing with what we're used to is very often the preferable choice where the costs of switching are high and the benefits unclear.

I actually think that infix notation has some advantages, especially for associative operations {e.g. if (G, +) is a group and a, b, c \in G, it's much more natural to write a + b + c than to choose between the equivalent (+ (+ a b) c) and (+ a (+ b c))}.

But undeniably S-expressions are superior from a teaching point of view as they don't hide the "true" structure.

Even though I don't dislike infix at all, it's hard to argue the reason most people prefer it isn't that they're just used to it.