There's already a thread on it, but it definitely qualifies as a bug and will hopefully get more attention here.

https://forums.galciv2.com/318189

Basically, the Drengin, Yor, and Korath AIs refuse to expand beyond their home sector.  They'll take planets that are very close by, but that's it.  This isn't due to any lack of scouting, lack of habitable planets, or lack of colony ships.  I've seen the Drengins refuse to colonize a planet in the very next sector, despite having 7 colony ships parked on their home planets.

The problem is easily recreatable.  Start up a new game and just keep hitting "end turn."  You'll see other races expand their influence, but these 3 races will just turtle up.  Even the Arceans, who have that nasty speed penalty, manage to expand further than the Drengin, Yor, and Korath.