My wife and I finally watched "Indiana Jones and the Kingdom of the Crystal Skull" the other night. What a disappointment! The movie was one part Indiana Jones, one part Close Encounters, and one part Ocean's 12. We couldn't understand why a great story teller like Steven Spielberg would sign on for such a poor story. From watching an interview with Lucas and Spielberg here seems to be the logic behind such a bad decision:

Demand was strong - everywhere they went, people kept asking, "When is the next Indiana Jones coming out?"
Keep the legacy brand alive - Spielberg wanted to make it for those that never saw any of the other Jones movies.
Lucas drove the alien (oh, excuse me interdimensional being) and they went along with it.

Here's a link to Spielberg talking about the movie:

The real moral of this movie is strong talent can't overcome a bad decision.