for several reasons:

1. It's the Capital or a very important city of my arch enemy thus a particulary powerfull and annoying one.*

2. I Have no intetion of holding the city in the next few years, in this case I also demolish most buildings

3. The public order-head is red when I conquer it and it is not one of my expanded homeland provinces. - depends how I roleplay sometimes this means lvl 1 government sometimes 1 and 2.

4. I had a bad day, DIE CIVILIANS DIE!!!!!!!!!!

*if their Capitol/prominent city is within my homeland I just expell or occupy them.