I'm not the author, but I just picked a Flask / Backbone stack over Django (after initially starting with Django) because so much of Django didn't seem to fit. Django wanted me to make a server side template based CMS. All I wanted was something where I could make a server that talked mostly JSON to clients. (My web application controls and displays results from using some custom hardware and will serve at most tens of users.)
Also, because I don't need teh web scalez, I'm using Postgres. For fear of making things completely undevelopable for hires outside Silicon Valley and NYC, I passed on stuff like Coffeescript and SASS.
