Making movies is a notoriously complex business. It might seem easy on the outside, but there's an extraordinary amount of work and organisation that needs to go into every production. Assembling a huge team involving actors, directors, writers, producers, photographers, and so much more isn't easy, and there's also the question of rights, marketing, funding, and more.
With so much to do and so many possible pitfalls, it's no surprise that some films get stuck in 'development hell' for many years. When this happens, a movie project is forced to a standstill, often passing between different studios, having its script rewritten multiple times, undergoing drastic changes, and making no real progress towards actually coming out in cinemas.
Some of these projects never actually come to fruition. There are movies that have been stuck in development hell for years and years and may never see the light of day. A movie version of The Jetsons, for example, has been touted for decades, and sequels like Austin Powers 4 are said to be in the works but may never actually happen. Fortunately, some movies do manage to find a way out of development hell, and they often turn out to be worth the wait.