Because most people have no idea what covers are, and unless you collect postage stamps the difference means nothing to you. If you'd asked me before I met a friend who collects, I would have assumed that using a stamp would render it worthless.

The article's talk page actually has a similar discussion over whether to call them stamps or covers in the title, so although they come down on your side its hardly a given.

I still don't understand what a cover is. Is it the cancellation stamp?

A cover is essentially an envelope (it got it's name because it covers the contents), usually stamped and those stamps cancelled.

Here's some examples of fancy covers:

https://store.usps.com/store/results/collector-s-zone-first-...

According to https://en.wikipedia.org/wiki/Event_cover they are "decorated, stamped and canceled commemorative envelopes"

Essentially they are envelopes with stamps and a marking over the stamps that prevents the envelope and stamps to be used.

https://en.wikipedia.org/wiki/Cancellation_(mail)

I think most people would not understand the technical term “cover” while “stamp” clues them in that is is likely somehow postal related.

"postal covers" (the article title) would seem to handle that.

in my 40+ years of living in english speaking countries and sending thousand of pieces of postal mail over that time, i've never heard the phrase 'postal cover'. 'postal' is the only clue that indicates it's ... to do with 'postal' stuff.

I agree. This is a technical discussion forum and we should use the correct terms.