Probably greed combined with a fee.
There's legitimately a licensing fee required for this thing, as there would be for Capcom re-releasing Aliens vs Predator, or the Dungeons & Dragons duology.
But at the same time, they're unwilling to eat the extra overhead to keep it priced at $15.
Having said that, people really don't appreciate the value a lot of $15 releases offer. 3rd Strike Online - problems which have been and are being patched aside - was a hell of a well done package and great value for that price.
The real problem I have with this release, isn't the price. If this had been an Iron Galaxy style port, chock full of features and extra content, $20 would be a good price to get such a niche game re-released with online play.
Unfortunately, there are indications this is a dodgy port, and not an Iron Galaxy quality package. We need to know more, but that's the real problem IMO.