This site seems to have a rule that "questions that are only about one site should be asked on the site's own meta", and such questions are nearly always marked "off-topic" here. However, I don't understand why this rule exists.

I checked the help center here, and I found this help page which explains the purpose of this site, but it doesn't say why (or that) questions only about one site aren't accepted here.

Alternatively, I asked my site-specific question here after I wasn't able to ask on the site's meta, but my question was still deemed "off-topic" even though I clearly explained why I wasn't able to ask there.

Why are such questions considered off-topic here, even in cases where I wasn't able to ask there?