Because private cars aren't carrying 300 people
But if you are after preventing deaths it would be far more productive in cars given the numbers of accidents and fatalities. There is no recording method in them so much remains a mystery.
A live video would not be of much benefit, what are you recording? The backs of two guys/girls, would the definition be good enough to see instrument indications? The only reason for this would be to know the plane has crashed, not why. We know the plane has crashed as it did not land. Sound recording is already there, not live but we will know soon enough and that is far more valuable than a video of a cockpit environment without much definition. The benefit would be far outweighed by the cost and misuse by companies.
Lets focus on something sensible when the findings are found. Spend the money on better identity checks, training, maintenance or whatever it is deemed caused this sad event rather than ludicrously expensive high definition multiple live feeds from every possible angle of an aircraft in flight. Its the knee jerk reactions that make everyones life harder, not safer. How am I safer not being able to take nail scissors onto an aircraft when I can purchase a large glass bottle full of flammable liquid to take on the flight with me? I could smash that bottle over someone, set them on fire and stab them. I am all for implementing better security and surveillance measures if they make sense but live camera feeds are not it.