[recycled sixtie]

I would think that if you had too many cameras on the outside this would cause drag
and also there is the expense of installation and maintenance. Also you would need many cameras to cover all the outside of the aircraft.

As an aside way back they used to have cameras in the cockpits of one airline so the passengers could see what was going on in the cockpit. This was discontinued I would think because if anything goes wrong with the aircraft the passengers would get a first hand view of what was going wrong.