TLDR: probably a lot of people continue using the thing that they know if it just works as long as it works well enough not to be a bother.

Many many years ago when I learned, I think the only ones I found were Apache and IIS. I had a Mac at the time which came pre installed with Apache2, so I learned Apache2 and got okay at it. While by release dates Nginx and HAProxy most definitely existed, I don't think I came across either in my research. I don't have any notes from the time because I didn't take any because I was in high school.

When I started Linux things, I kept using Apache for a while because I knew it. Found Nginx, learned it in a snap because the config is more natural language and hierarchical than Apache's XMLish monstrosity. Then for the next decade I kept using Nginx whenever I needed a webserver fast because I knew it would work with minimal tinkering.

Now, as of a few years ago, I knew that haproxy, caddy, and traefik all existed. I even tried out Caddy on my homelab reverse proxy server (which has about a dozen applications routed through it), and the first few sites were easy - just let the auto-LetsEncrypt do its job - but once I got to the sites that needed manual TLS (I have both an internal CA and utilize Cloudflare' origin HTTPS cert), and other special config, Caddy started becoming as cumbersome as my Nginx conf.d directory. At the time, I also didn't have a way to get software updates easily on my then-CentOS 7 server, so Caddy was okay-enough, but it was back to Nginx with me because it was comparatively easier to manage.

HAProxy is something I've added to my repertoire more recently. It took me quite a while and lots of trial and error to figure out the config syntax which is quite different from anything I'd used before (except maybe kinda like Squid, which I had learned not a year prior...), but once it clicked, it clicked. Now I have an internal high availability (+keepalived) load balancer than can handle so many backend servers and do wildcard TLS termination and validate backend TLS certs. I even got LDAP and LDAPS load balancing to AD working on that for services like Gitea that don't behave well when there's more than one LDAPS backend server.

So, at some point I'll get around to converting that everything reverse proxy to HAProxy. But I'll probably need to deploy another VM or two because the existing one also has a static web server and I've been meaning to break up that server's roles anyways (long ago, it was my everything server before I used VMs).