There is a great deal of resistance to video from pilots and crew, who incorrectly believe that it would be used mainly against them. Police officers were opposed to dashboard cams in the same way until they discovered that video was just as likely to help them as it was to hurt them—video is a neutral observer, so people doing stupid things are exposed, but people doing smart things are vindicated.
There are also cost issues. The cost of video is high and its advantages are low, since crashes are very infrequent in commercial air travel. It would certainly be invaluable in some crashes, but there are too few of those to justify the cost (indeed, one could argue that there are too few crashes to justify the cost of a black box at all).
The NTSB wants video because it wants as much information as possible about every crash, and video would be tremendously useful in certain situations, since it could show exactly what the pilots were doing in the cockpit. But the NTSB is concerned only with safety, whereas airlines are concerned only with money, and pilots are concerned about hiding their incompetence (at least if they know that they incompetent).
Weight and size issues no longer apply with modern technology, but the obstacles above are more than enough to keep video out of black boxes for now.