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

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.