Most videos of miniatures rules aren't done by the publisher, but rather by enthusiasts, magazines, or other independent reviewers.
It's probably just that Empire V appeared 31 years ago and thus pre-dates the existence of YouTube by about seventeen years. It's likely that the game appeals to a certain demographic that might not be video-production-savvy.
Making videos is quite a bit of work, and it takes longer to explain something in speech and with images, than it does to explain it on paper. So I imagine that for a game with as many steps, charts, tables, etc, as Empire, it would need to be a pretty long video or series of videos. There probably just hasn't been any incentive to do that for a game that was printed so long ago.
Not to mention: among Empire enthusiasts, there are some divisions regarding which edition should be used. If you did a video about the "wrong" edition, you'd have to put up with all their snark.
Maybe if there really is a new edition coming, that would be the right time to do some videos and help promote it. There's not much point in trying to promote a 30-year-old game now; even if the videos got somebody interested, they'd have a hard time finding a copy to buy.
looks at some new terrain products, which use space age technology!
