New Brevoort Formspring

It differs from instance to instance. But on longer runs, there tends to be an overall feeling, a zeitgeist, of which even the sales are a component part. But more importantly, there’s a sense that a particular creator has done about as much with a given series as they can, and both they and the title would be likely to become re-energized with a change. Sometimes it comes from a writer simply realizing that they’ve come to the end of their own particular line.

Typically, though, writers don’t campaign for each other’s jobs. That’d be pretty bad form, especially among people that they would then have to continue to work with. There’s a difference, for example, in expressing a desire to write Spider-Man at some point and doing the kind of thing you suggest in your question.