Data recorded all the time the aircraft is powered up would overwrite flight data with useless ground data.
That's EXCATLY what does happen, frequently, following serious incidents to aeroplanes that then remained powered on stand - subsequently causing a hinderance for the accident investigators who then find no trace of the incident data itself.
I had a similar experience myself a year ago when, following a forced landing in a remote area in a helicopter (in jungle) I subsequently powered up the APU to use the satphone to talk to base, several times. Subsequently, when we listend to teh CVR to see exactly what the crew response to the problems in flight was, we found the 2 hour recording started 26 seconds before the first indications, had 5 minutes in the air and the rest was 1:50 of telephone chat! At the time, pulling the CVR cb was not top of our priority list!