I asked this question ages ago and it was pointed out that “sub” isn’t a reddit specific term. It’s been short for “subforum” since the first BBSes, so it’s basically a ubiquitous internet term.
“Sub” works because everybody already knows what you mean and it’s the word you intuitively reach for.
You can call them “communities” if you want, but it’s longer and can’t easily be shortened.
I just call them subs now.
Reminds me of the tyre store that spreads tacks on the road 100m away from their store in the oncoming lanes.
People get a flat, and oh what do you know! A tyre store! What a lucky coincidence.