HTTP 429 instead of HTTP 400 for quota overuse

Is there a reason for returning a HTTP 400 Bad Request on request quota overuse instead of HTTP 429 Too Many Requests?

Technical limitations caused by the version of Apache http that we are currently running on our backend servers prevent us from returning HTTP 429 at this time, however we intend to switch to it once that dependency is eventually updated. No timeframe is currently available for that however.