there were two reasons - one more practical, the other one more idealistic.
the practical reason:
Last year I worked 3 days at a law firm and 3 days a week at a university chair (yes, there are only 5 work days in a week, and that was part of the problem :-), which was becoming a considerable strain on my time and energy. But in addition I also tried to fit in time for writing my doctoral thesis and a couple of other activities I enjoy doing. Naturally, the doctoral thesis never made it on my schedule - with it being "just" important but not urgent - and my extracurricular's suffered as well. So after a year I realized I had to quit one of the two jobs in order to make time for my thesis and at least a hint of free time. And so, for practical reasons, I decided to quit working in a law firm and focused on my university job. I figured that if I really wanted to, I could later still work my a** off in a law firm for the rest of my life, but I wouldn't be able to go back to a university job as easily.
the idealistic reason:
During my year at the law firm I initially had great fun working on individual cases and solving a client's problems. But after a while I realized that work became rather repetitive (though on a high level) and overall was not as gratifying as I had expected. I felt more at home at the university chair, where you are (mostly) independent from an individual client's wishes (and idea of "justice"), can focus on finding the "right" or perfect solution to a problem, take long-term goals into consideration and - most of all - have the freedom to determine your own work and research agenda.
I am still glad I gained considerable experience in a law firm though, because now I know that if my current career plan fails, I can always go back to a law firm and make a living there.