Quote Originally Posted by OhOh View Post
Project modifications/creep or poor estimates of development/budget availability?
A bit of everything. One very welcome feature creep. James Webb is a deep infrared telescope and the sensor needs to be kept very cold. Initially the plan was to cool it with a stock of liquid helium. Once that runs out the telescope is almost worthless. They have replaced it with an active cooling system that can run until it dies of old age.

But the most important thing, NASA gave the contract to a small but promising company. Soon after the contract that company got bought by Lockheed Martin. Then the disaster unfolded.