We support 4 different ones to give folks choice. Some folks want/need features that aren't available on one vs the other (traefik has a ton of features, caddy is simple to configure, nginx has a ton of documentation) so it made sense from that perspective. It was also easy to add once I had the pattern going (though the default has stayed nginx).

One of the main features of Dokku is it's extensibility. You can cut one part out and replace it with another quite easily, and proxying is an example of that. I think that flexibility allows folks to use it in more situations than one otherwise would, though at the cost of being more difficult to maintain (and harder to have cohesion between parts of the system at times).