I searched this on many places. Only valid reason I can find is the extreme temperatures on the out of the plane. Also, from what I can find airplanes generally have camera near landing gear to aid taxiing.
Besides technical reasons, there is a legal angle...
> Pilot unions and other groups have long opposed cockpit video cameras, arguing that images or footage may be misused by accident investigators, prosecutors or news media.
> Pilot unions and other groups have long opposed cockpit video cameras, arguing that images or footage may be misused by accident investigators, prosecutors or news media.
https://www.google.com/amp/s/time.com/3845234/cockpit-video-...