snowkatt: depends on if the developers liscense holders and publishers are interested
I don't think the license holder is the problem in this situation (in one of the few rare instances), as GOG has already released 4 Warhammer games here. The problems seem to be two in the case of Dark Omen:
1) Who owns the game's rights? Is it EA (who was the publisher) or Mindscape (the developer)? If it was EA, it's not exactly surprising why they wouldn't bother with an old licensed game. If it was Mindscape, then it's very possible that GOG successfully acquired the game's rights, but this leads to the second problem...
2)... technical issues. Since Warhammer: Dark Omen is a late 90s Windows release, it's very likely that it's suffering from issues that makes its rerelease difficult. While I can't personally say whether this is really the case, the previous game in the series (Shadow of the Horned Rat) seems to be having problems launching in modern systems (just read the reviews for the game here).
Matewis: Always wanted to check it out, but I don't think I ever saw a copy in a store. It seems Warhammer fantasy is a bit underutilized in gaming come to think of it. Apart from the MMO I only know of Mark of Chaos and the upcoming total war game.
There is
Shadow of the Horned Rat which is the game that came before Dark Omen. There is also
Mordheim and
Vermintide on Steam.