There are traders who are just distracted by their main job. That's why if they profit a bit from trading, they just quit any other job they have.
Well it is maybe because trading requires lot of attention. You need to be focus on every movement of bitcoin. As for me I just put a time or manage my time on when I will earn money. Since I can do both, I do bitcoin when I have free time. It is a fulfilling feeling to learn that I am able to earn twice.