Apparently one of the main reasons it took so long to make this film was that Ford and Spielberg were not interested in the unsubtle depiction of alien invaders. "No way am I being in a Steve Spielberg movie like that," Ford told Lucas. Lucas wanted to call the new film 'Indiana Jones and the Saucer Men From Mars' but had his idea firmly rejected by Ford and Spielberg as the three men struggled to reach an agreement for around 10 years.
So what the hell changed?! because the film we got is pretty much exactly the same as the original Saucer Men draft written in the nineties.