Math Overflow was created as a Stack Exchange 1.0 site, which means essentially that SE hosted the software, but everything else was done by the users that initiated the site.
A few years ago, SE stopped supporting the 1.0 sites, and Math Overflow decided to join the Stack Exchange network. They're still somewhat special, e.g. they have the right to leave the SE network again, if they would want to.
A few years ago, SE stopped supporting the 1.0 sites, and Math Overflow decided to join the Stack Exchange network. They're still somewhat special, e.g. they have the right to leave the SE network again, if they would want to.