For a few reasons:
1. The last Dark Forces game came out in 1997. At this point, so much time has been removed and gaming changed so much that it might as well be a reboot.
2. LucasArts no longer exists as it was. The Star Wars games are now being produced by Electronic Arts, who are focused at the moment on rebooting the Star Wars Battlefront franchise.
3. There's an orchestrated effort by Lucasfilm to put everything into one unified continuity, with all of the old Expanded Universe being jettisoned from that continuity (leaving only the core films, the CGI Clone Wars cartoon, and anything published after about mid-2014). The Dark Forces games are no longer part of the established canon, so it wouldn't make sense to create a sequel to old non-canon games (unless, like stated previously, they just reboot it).