The answer is actually a long one. The original pitch and first couple scripts for Aliens 3 were more like Aliens. Ripley was meant to be pushed aside while Hicks would of taken the starring role. Hollywood meddling and writer/director disatisfaction sunk it though. Then came quite a few more scripts, with action (Including an incredible one of a proto "Riddick" escaping from a prison in space, scenes of which were word for word in both Pitch Black and Chronicles of Riddick).

However after script after script was turned down, Hollywood executives decided they needed Ripley to propel the franchise forward. Problem was Sigourney Weaver didn't want to do another film. In came Vincent Ward with his "Wooden Planet/Monk Script" which finally changed Sigourney's opinion.

While the original plot probably would of made for a tense movie, it would of been obscenly expensive to shoot. So they took the basic outline, slapped in the prison plot from the "Riddick" script, changed the monks to prisoners and gave it to a young and upcoming director. Hence Alien 3 and why it's so different from the proceding films.