Why do some professors with PhDs leave their professorships to teach high school?
I have done this so perhaps can answer your question. I was a tenured full professor of physics at a state university but currently teach high school.

There are several reasons I chose to do this. 


Most importantly, I love teaching and am great at it, while I dislike the constant drudgery of research and will not be winning any Nobel Prizes. I never have to hit my head against the wall of grant competitions or peer review.
There came a time when I needed to solve the Two-Body Problem and the high school option made me far more mobile.
I get more time for myself. Classes generally require only about 5 minutes of prep time each, if that. The time between periods is sufficient. Grading takes less than 10% of the time it took at college because of the level of study and can often all be done in class while students are taking a quiz or test. When school is out, I go home and forget about work. There is no constant e-mailing from students wanting help and extensions.
My interactions with high school students are closer and more positive than those with most of my college students. I never learned most of my college students' names. In high school, I have a relationship with every student.


The down sides are minor. The pay kind of stinks compared to what I used to make, but the free time makes up for that. I sometimes have to loop in the Headmistress when making grade decisions that involve teacher's discretion, which grates on my sense of academic freedom. But I think I can deal.