Because (? is invalid syntax on its own, in any (?X pattern the X is unambiguously special or incorrect, but never litteral. perlre however does talk about extending the quantifier syntax.

like making the lower bound of a quantifier optional

And any new syntax that's not a modifier can be added with a (?X pattern. That's why I concluded that disallowing unescaped litteral { was probably an opening toward new quantifier syntax (because { only introduces quantifiers). I thought of something like /.{{!($_ % 3)}}/ instead of /(?:.{3})*/ (it's harder to read though ...), or even "3ABCD" =~ /(\d)(.{{$1}})(.*)/; #(3,"ABC", "D"). But that's wild guesswork

PS: How do you like all those unbalanced left tokens? ^^"