Flask is inherently simpler/smaller than Django. For doing simple/small web sites its a naturally good fit. However, due to its simplicity it needs to be significantly augmented if you intend to build a more complex web app with it.
Augmenting Flask with a data persistence layer, a RESTful API system, a more robust templating system, and any of the other infrastructure tools you'll need (user auth, session management, xss protection, input cleaning/validation, etc.) is quite complicated. If you go through those steps you'll basically just end up with something like what Django gives you right out of the box. Except that Django's components are well integrated with each other and fully tested, and your custom built system won't be.
Augmenting Flask with a data persistence layer, a RESTful API system, a more robust templating system, and any of the other infrastructure tools you'll need (user auth, session management, xss protection, input cleaning/validation, etc.) is quite complicated. If you go through those steps you'll basically just end up with something like what Django gives you right out of the box. Except that Django's components are well integrated with each other and fully tested, and your custom built system won't be.