Mainly because it is recommended [1] to have only one h1 on a page. And my pages sometimes contain multiple articles.

It feels natural to me to have h1 be the site name. And have h2 be the content title. And it happens to work well when multiple unrelated articles or other kinds of content are on the page because then I use h2 for each of those.

[1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/He...