No organization wants to do this, unfortunately. Twitter and Instagram are “free”. Running servers costs money, made more expensive by the choice in the Mastodon design to not support virtual hosting on a per-domain basis. You have to run (and admin) a complete new instance of Mastodon for each domain you wish to support.
It would be really really nice if it was easier for small organizations. For big organizations they're already in the business of running their own websites and this isn't anything new.