In addition to the other commenter and their great points, here’s some more things I like:

• ressource efficient: im running all my stuff on low end servers, and cant afford my reverse proxy to waste gigabytes of RAM (kooking at you, NPM)
• very easy syntax: the Caddyfile uses a very simple, easy to remember syntax. And the documentation is very precise and quickly tells me what to do to achieve something. I tried traefik and couldn’t handle the long, complicated tag names required to set anything up.
• plugin ecosystem: caddy is written in go, and very easy to extend. There’s tons of plugins for different functionalities, that are (mostly) well documented and easy to use. Building a custom caddy executable takes one command.