There's two things keeping Terragen off the small screen (and big screen to a degree).
1. Miss-conceptions on the part of the design community, thinking Terragen is a small hobby or very specialized market. "Where on earth could we find someone who can even use this program?"
2. Long render times.
Both these issues are being addressed. If you look back through the archives of this forum and take a look at the images produced three years ago and those cumming out now, I mean wow!! how far we have come! We can now produce stunning images of better quality than the competition in a reasonable amount of time. Most studios will have their own render farm and the rendering efficiency is getting better all the time especially with the "bang for buck" cost of each render node dropping all the time.
It won't be long no before Terragen will be used as just another tool in the videographers tool box

Richard