Because that's what APL does: https://www.jsoftware.com/papers/EvalOrder.htm

It also simplifies things as no need to implement BIDMAS

Think "normal" call syntax like

  foo(bar(baz(42)))

and then remove the superfluous parens

  foo bar baz 42

The expression is evaluated from right to left.

Now, let's make two of the functions into object members:

  A.foo(bar(B.baz(42)))

Remove the parens, extracting the methods from their objects, instead feeding each object as a left argument to its former member function:

  A foo bar B baz 42

This is normal APL-style call syntax; right-to-left if you want.

Oh now I see it, it sorts of reminds me of lambda calculus

Thanks for asking. I would guess you're asking as someone that spent years learning math notations and being taught BODMAS operator precedence. The funny thing is that if you took a 3 year old child and taught them right to left it's actually more natural than multiplication before addition. Array languages often take a fresh first principles approach rather than regurgitating common learnings. This does mean programmers from other languages can find it more confusing than total beginners.