This has irritated me for about a decade! Probably unreasonably.
Rasberry Pi was not the first ARM dev board with linux, and most of the "Make your Pi do X!" recipes out there would more reasonably be described as "How to set up your linux server do X", but that's not cool, and had no Pi, so ...
Grrrr mumble mumble, yes I know I'm an old curmudgeon.
I think the reason is that is that Pi has brought Linux in the flavour of Debian to the general public, many won't understand the concept or importance of an OS or how it can be portable; It's Raspberry Pi's "Software".
The second and more likely reason is that Raspberry Pi are keywords that help get you in the hands of your target audience, I'm guilty of it on my blog. If you're running a Debian server on x86, you're probably not the target audience for a "simple" VPN tutorial.
A while ago someone had a "how to set up your own Alexa on a Pi". It was just using Java, so I tried it on Windows, and it worked. I commented so on the guide, which was hosted on Github, and I got many many replies asking "Can you tell us how to do it?".
It annoyed me that these folks have a Github account but can't figure out things without step-by-step instructions...
Rasberry Pi was not the first ARM dev board with linux, and most of the "Make your Pi do X!" recipes out there would more reasonably be described as "How to set up your linux server do X", but that's not cool, and had no Pi, so ...
Grrrr mumble mumble, yes I know I'm an old curmudgeon.