I can think of one reason via experience. I have a law degree and practiced law for 13 years before stopping when we had our second child. My wife is a physician and we could not both maintain our careers and fulfill our parental responsibilities. I taught at a small school for two years which fell apart in 2015. I then got a masters in US History. Since then I have been looking for work and have applied for dozens of jobs. I cannot even generally get an interview even for middle and high school positions. It is difficult to get substitute appointments. I was told for one long term sub position (January to May) there were over 100 applicants. I went to a job fair at the University of Virginia which was overflowing with history and English majors. I believe there were more applicants there than teaching positions in the entire state (a little exaggeration here, but not that much).
I keep hearing about the shortage of good teachers, which is true for math and the hard sciences. But in the humanities there is a glut. I can see why so many are depressed. Fortunately for me, I get a rare opportunity, the ability to spend a lot of time with my young children. My youngest is 5 now though and will be in school fulltime in the fall. I would like to get back to fulltime employment but that seems unlikely in the near future. I created a blog (some articles posted here) because history is such a fascinating subject to research and write about. It is discouraging to say the least that with a passion and significant educational background that I can't even get interviews.