Oversight + incompetence is the usual reason.
The CVR Erase button is typically in the cockpit on the overhead panel, next to the CVR Test button. The CVR will be "erased" if the button is held (usually for 2 seconds) while the aircraft is on ground and parking brake is set. The CVR cannot be erased while the aircraft is flying.
Some countries / jurisdictions do not allow CVR Erase functionality, so the button is either disabled or completely removed (factory option to not have the button). I don't know what the regs are in China.
Dirty little secret: by design CVR Erase is not a "secure erase". Meaning, it is possible for the authorities to use data recovery techniques to reconstruct the CVR recording, even if a pilot "erased" the CVR. Once the recording has been overwritten, however, recovery is no longer possible.
As an aside, on the A330 the CVR circuit breaker is not accessible from the cockpit (it is in the avionics bay).