To the extent that true, it's because of the weird historic moment between the mid 1990s and now where essentially every industrially popular general purpose language is from the C branch of the Algol family.
It's not really the whitespace (which is clear in semantics), and other than the lambda slash and the infix ticks, the whole thing is pretty clear from a Lisp background. (Well, I'm familiar enough with Haskell now that I'm not filtering it through some other language, but the similarity to Lisp is what helped it start to click early on for me, even though it's an ML descendant and not a Lisp descendant.)
We're actually starting to see more gains for non-Algol descended general purpose languages, so maybe the syntax shyness of the last generation or so of programmers will soon be a thing of the past, because knowing multiple languages won't so invariably be knowing multiple members of the same syntax family.
Perhaps I've been using Haskell too long but that code looks very clear to me.
thenP :: P a -> (a -> P b) -> P b
m `thenP` k = \s ->
case m s of
Ok a -> k a s
Failed e -> Failed e
As to the comment re: control / python, I'm sorry but it's so obvious to me the syntax would be more legible to most developers (since "most" use Algol descendent syntax languages) even if done somewhat poorly, I don't feel like taking the time. I welcome you to prove me wrong, I'll gladly stand corrected if so!
Take this example of Haskell from their docs:
It doesn't matter how good you are in [insert almost any common language], you'll really struggle to understand that code.It's my same objection to things like Coffeescript, if you think you are so much more productive not typing semantic tokens like ()s or {}s, then you need to lay off the coffee and get some sleep as you're clearly dillusional.