I think most people when they keep doing the same job for a while, lose the 'hunger' and their productivity/quality suffers. Add success to the mix and the slide is even more slippery. And since the director is so dependant on the material available to him/her, its really hard to maintain the drive and the quality of the other people around him/her.

Now from the 70's:
Peter Bognovich, John Carpenter & William Freidkin had drug problems after they had their string of great movies.

There are several factors that lead to this.

Most artists have a creative peak. Some may have more than one. But no one has a plateau of artistic creativity--a career-spanning high. This is the nature of an artist. In the quote mentioned above about Lou Reed and David Bowie, arguably two of the most influential men in rock, both had their fallow periods. Bowie, at least, seems to have hit another high, but look what he had to go through to get there.

There are many reasons for this. Younger people are hungrier, and can produce more. Secondly, they're more willing to experiment, and try different things. Spielberg, looking back on Jaws, said, "I was either fearless or completely stupid." The point being, if he did Jaws now, it would not be the same movie at all.

Secondly, the stuff you like about films made by directors in a certain period of time may, to them, become ideas/techniques/styles that start to feel tired to them, so they try something else. Unless the new thing is really amazing, you'll have people saying, "Oh, he'll never match the level of his earlier films now," and if they try to go back to the old style, it's rarely seen as a return to form, but usually as an artistic retreat.

And, finally, as someone said, they have to pay the bills and many get involved with drugs or just get a big head.

Kubrick managed to avoid most of these pitfalls, but in many ways Kubrick worked more like a novelist than a filmmaker.

Burnout.