Flask gets you up and running extremely quickly, but as soon as you need to do anything moderately complex you need a heap of extra packages or code. Django comes with the kitchen sink included, but has a much steeper learning curve to understand how all of the parts for together (e.g. there may be 3 or 4 files you need to edit to make a change to something).

I use both, for very different applications.

There are also some packages for Django that make it more flask-like (e.g. importd) but I've never used them in real life.