That’s a good question @iamCristYe. It was raised before here: Editing nginx lms and cms.conf - #5 by MaartenGH
Basically, I agree that Caddy and Nginx serve somewhat similar goals. But Caddy is a much thinner layer that does only SSL termination. Because users may want to use a different proxy for SSL termination, there is a simple mechanism to completely remove Caddy from a working Tutor deployment. But in that case, we will still need a web server to redirect requests to the various containers. So we decided to keep nginx. In addition, there is the question of backward compatibility of nginx patches and plugins.
Still, there might be a way to keep Caddy and remove Nginx, even in the case of custom SSL termination. I’m open to suggestions here.