I've known many bioscience candidates, and nearly all of them had to quit and take their masters instead. The reasons- burnout, babies, and bank accounts (they need to start working to support their new family).
But more than any other factor- low morale. They are surrounded by folks with more brains than manners or personality, including professors, underlings, and co-equal lab partners... very unpleasant people.
I don't know why I should be amazed at the nastiness of some academics, but I always am when I hear these stories. What is the purpose??
I've been fortunate that my program is full of decent people who treat the doctoral students decently. I may be the only happy PhD student on the planet, but I truly enjoy my studies and my professors.
PS - I came to academe from a career as a lawyer. Trust me, it's hard for me to be happy or enjoy anything. Perhaps compared to law school and practicing, anything looks like fun. : )