Because Volition and Outrage were trading off duities. Parallax did Descent 1 and Descent 2, then they split into Outrage and Volition. Volition did Descent Freespace and Freespace 2 while Outrage worked on Descent 3. Then, it became Voliton's turn to do Descent 4, but they decided to basically kill the series and make RF. Outrage is now basically dead as far as I know. Last I heard, Volition was going to release what they had done with D4 to the public to let the Descent community develop their own D4.
It's been awhile since I heard anything though.
Vostok 7
Whatever. Volition could very well work on DC4 as they have released nothing but crap since FS2. RF sucked, Summoner is a mediocre series, and so are both respective sequels.