Chalk it to more of Lucas' hit-you-over-the-head exposition. If the Lucas of today made the original three, they would have been titled:
Indiana Jones and the Raiders of the Lost Ark of the Covenant
Indiana Jones and the Temple of the Sankara Stone-Worshipping Khali Cult
Indiana Jones and the Last Crusade to Recover The Holy Grail of Christ's Last Supper
Consider how much cooler the following two titles could be for the new film:
Indiana Jones and the Lost Kingdom
Indiana Jones and the Crystal Skull
Either one flows better, is more concise, and doesn't smack of a bloated old hack fucking up his best work.