We didn't get video on DSLR's because there was a massive clamour of photographers asking for it. It was a happy accident, and it'll follow whatever quality standard comes off the sensor.
Camera manufacturers wanted to appeal to all those digital P&S shooters who were used to a big rear screen (and stupid shooting position but I digress) so they fudged a 'live view' shooting mode.
Once this was done, it was a simple step to record the feed coming off the sensor - thus DSLR video was born - there wasn't a large number of videographers lobbying for video from stills cams, but when they discovered it they were hooked. The only 'improvement' required only for video is frame rate.
As for it being a small market

, there's not a week goes by that I can't spot some DSLR shot video on TV, on a wide variety of shows, it gives a very different 'look' to even broadcast quality video cameras. The market for HQ video on DSLR's is massive, you need to understand how cheap it is in comparison to HQ video cam's.