Originally Posted by Parker Wayne
The problem is that none of the movies franchises were stable for the third film. Either studios begin to tamper with the final product, or the director leaves.
Spiderman 3 - Raimi was forced to add villains and had a hard time making it dark on top of other crap he put in it
Batman Forever - Tim Burton leaves and is replaced by Schumacher. Burton's original script for the film is heavily tampered with.
X-men 3: Brian Singer is replaced by Brett Ratner. Do I need to say more?
Superman 3: Richard Donner was already gone and producers once again ripped this film to shreds.
Blade: Trinity - different directors with different visions had done all three Blade films so it was bound for trouble.
It takes one united vision (and near complete creative control) to create a great and successful trilogy. It's as simple as that.
You're absolutely right. There needs to be less interference, and more stability. I wish I could add more, but you already said it better.