Upvote:2
Most likely because it doesn't offer any real value to the consumers, apart from a few minutes of entertainment during landing/takeoff. It would also become repetitive very quickly for someone who flies often.
Therefore such an improvement would be placed well below other entertainment features such as on-board movies, music, games, Internet and phone calls.
The airlines who already have it most likely had the option to install it at little to no cost. For example, if the airplane had the cameras hooked to the internal communications system for the pilots to use, it would be trivial to allow the passengers to view the cameras as well.
Upvote:3
In the US this has been offered on one of the flights but had since been removed.
The flight in question is American Airlines flight 191 that crashed in Chicago in 1979, has been documented in the Air Crash Investigation Series(43:50 seconds in). Basically it's not a regulation by any regulatory body but just an airline choice not to show critical or non-critical phases of the flights on most if not all airplanes in the US.