I would guess a fairly big factor is because generally the game logic which runs of the processor doesn't degrade well. With graphics you can lower resolutions, change texture sizes and add additional lighting effects which are optional so the game just looks a bit worse but plays the same. Trying to do the same with game logic is much harder, maybe some adaptive AI could be made to play better on faster hardware plus some extra graphical effects probably need some extra processor time but these changes would be much less.

Then the developers want the game to be playable on as many machines as is feasible so they head for as low a target as possible which is a single threaded machine. Making it work on multiple cores then turns into a pretty difficult task for very little gain because of the above.

My big problem with Chronicles of Riddick wasn't any of the cinematography or plot or anything like that... It was the jump from a sci-fi setting to a fantasy setting.

The director's cut is actually worse, in my opinion, than the theatrical release.

During the big brawl on the prison planet there's a kind of explosion that kills a pile of badguys and knocks Riddick out. In the theatrical release this is some kind of energy pistol exploding for some reason. In the director's cut this is Riddick channeling the anger of all the dead people from his home planet.