The reasons are the usual: writing pays too little money, and you need to get a day job; you need health insurance; personal crises drain your financial and emotional resources; publishers are no longer interested in your work.
Rusch points out that many writers change their names to get away from bad sales records, or move into a new genre.
Or people simply lose interest in writing. They proved their point and said what they had to say. More painful, the creative springs can run dry, who knows why.
I think I (sort of) fit into the category of writers who became less visible, maybe even invisible. Back 15 or more years ago, I looked at the neat little report that the Social Security Administration sends on expected Social Security income; and I realized I had spent too many years working low paying jobs and part time. I did this in the theory that it would leave more time and energy for writing; and it more or less worked out.
But now I needed to build up my Social Security; and I needed to save some money. So I began to work longer hours, at somewhat better pay. I found these new jobs both interesting and draining, and I wrote less than before.
When I got laid off in 2009 and wasn't able to find a new job and formally retired, I finally had time to write. I've had to relearn writing. At first it was really difficult. I no longer enjoyed working on stories. Slowly -- very slowly -- I have gotten my creativity back. Maybe not all of it, but enough to produce new stories that I like and want to place.
And I've noticed that working on a writing career takes time and energy -- networking, corresponding with editors, reading contracts, proofreading stories before they are finally published. I realized years ago that freelance writers have two jobs: getting contracts and then writing the novel or whatever it is; and the marketing is as hard as the writing. I'm relearning that now.
Look over this post and notice how often I have written "job" and "work." Writing is a job. It is work.