Web & App Server
Nginx was another one of those personal choices where the choice was based on familiarity. But that isn't the only reason Nginx makes a powerful choice of a web server. It has a proven track record of one of the best web servers out in the market. Apache is another great choice for a web server.
We have knowledge and experience with Apache on other stacks in the past. We would be hard pressed to define why not Apache without some concrete details. We would just say this about Nginx over Apache. We preferred or found it more intuitive the Nginx way of configuring your web server as opposed to Apache's method.
Gunicorn was our choice of Python WSGI server after reading extensive comparisons, performance analytics, real world performance, other folks experience and comments. It's simple to install and configure. It's lightweight. It has a mention as part of the Django docs. If you really want to see comparison results by Cisco, here is a blog on it.