Actually, it was purely a decision to maintain some backward compatibility with earlier regex implementations that didn't treat curlies as metacharacters. In retrospect, probably a mistake, but traditional regex syntax is full of such mistakes, where rather than having a consistent backslashing rule, you have to put metacharacters where they don't make sense to match them literally, such as the infamous []x-] character class, which cannot be written in any other order, because the literal hyphen must come either first or last, and the right bracket may only come first, so the hyphen must come last.