Because:

1. I didn't really have the energy to job hunt while working 40 hours.
2. I thought things would get better. I thought I was getting better at dealing with things.
3. I just kind of fell into doing IT support and didn't have the education, training, connections, or experience to do much else.
4. I wanted to have more money in savings so I could move out of my parents house.
5. When I quit the job prior to that, I was unemployed for 6 months and it was one of the lowest points of my life, so I refused to quit again.

I ended up getting fired, spent the better part of 6 years unemployed after that (see point 3), now I work part time in food service so I only occasionally hate my job, I still live with my parents, I try not to hold illusions about #2 anymore, and #1 is still true but with less hours worked.