Because it's not part of CBS' corporate strategy. They are doing great with their police procedurals and sitcoms. Why should they divert energy and resources from their successful projects to dabble in sci fi, which CBS doesn't understand and has no interest in? One big reason why CBS is the most successful of the major networks is their discipline and relentless focus on what they know works. They don't experiment.why doesn't CBS test the waters and see what part of TREK lore really garners that much attention?
And although Paramount wouldn't have any control over this, direct to DVD Star Trek would erode the premium brand that Paramount has spent large amounts of money building back up, after being badly damaged through mis-management. I'm not sure if Paramount could put a stop to CBS's DVDs, but I'm sure they'd try.
Would Family Guy, Futurama or SG-1 movies have a hope in hell of cracking the top ten in annual box office? Of course not. So none of those are even close to being top-tier. And don't think of top-tier as being "quality" so much as in perception that it's a "big deal." There are plenty of brands that are based on crappy products, and plenty of great products that aren't strong brands.Overall, though, the general perception of direct-to-DVD is as AviTrek and I pointed out above: second-tier material, cheaply produced knock-offs and sequels... typical B-movie fare in a world that no longer has theaters which show B-movies. Maybe the success of the Family Guy and Futurama releases, and however this SG-1 movie does will change the perception, but it's a long, uphill climb.
McDonald's is a huge brand. Are their burgers any good? No. But the McDonald's name is a big deal that commands everyone's attention. It's really a perception game.
DVDs aren't bad because DVD movies are usually crap. They're bad because it makes you look like you're a loser who can't get your movie into theaters. Then if there's a movie with the same brand that you're trying to get into theaters, it's not going to be afforded the same respect, and then you've damaged the chances for your cash cow - the top ten box office champ - to make money, and for what? For the piddly amount you'd get from DVD? Not a good business proposition.