The biggest reasons for this are local variables vs. stack ops and parenthesized expressions vs. having to know the arities of the words called. You can add local variables to Forth, but afaik nobody did during its heyday; and "phrasing" your code with whitespace can help some on the latter issue, but it's admittedly not as explicit as parens.

(From another one-time Forther and more recent Lisper.)