He did explain why:

"This provides both future-proofing against later changes in ECMAScript (adding keywords), and simplifies the grammar for conforming parsers. Simpler grammars mean less opportunity for errors and non-conformant implementations."

If it didn't require quotes, currently valid JSON may not be valid in the future if a new keyword which was use as a JSON object key was used unquoted. And there's the added complexity of supporting both quoted and unquoted keys.