The problem is that video data is huge (from hundreds to thousands of times as large) compared to the ordinary telemetry that the recorders normally handle. As a result, they're having to develop recorders with many times as much memory than before. This isn't ordinary off the shelf memory due to the impact survival requirements, so packing all of that memory into the boxes takes some work.

There's also the issue of older planes. Many older airliners currently have flight data recorders that monitor from 7 to 15 telemetry points, things like engine power settings, altitude, etc. A more modern FDC can record hundreds of points for a much more accurate picture of the plane's condition. However, adding modern FDCs would require a significant engineering effort on those older planes. You'd have to determine which points to monitor, develop and install the transducers, add a lot of wiring, and do the calibration testing. Nothing on airplanes is cheap. Believe me - I'm a part owner in one. It seems the only thing on a plane that's reasonably prices is the air in the tires.