William has no clue what he is talking about. Feel free to ignore any of his posts.
For the record, Pumpkin's version of Warzone 2100 used DX 5/6.
Once the source was released, in order to make it crossplatform (Windows / linux / mac/ whatever) openGL was used instead of the proprietary tech (DX 5/6), which was later removed for obvious reasons.
As for Warzone 2100 not being "successful" that depends on what you mean.
We still get a ton of downloads, and the vast majority of those downloads are in it ONLY for the Campaign game, and skirmish.
The MP "community" isn't very friendly to so called "noobs" with people getting kicked out of games for being "noobs". So, it ends up being the same clique of players play against each other, instead of helping "noobs" become better...then they get bored, and disappear, and the community keeps shrinking.
That isn't to say that everyone does this, since there are some players that try to train these "noobs", but, there are just not enough of these kind of people to change things.
![Sad :(](./images/smilies/icon_sad.gif)