Because they know if they didn’t they would lose tons of users immediately. They’re betting that many of those users will slowly get used to the new site, or th

Because they know if they didn’t they would lose tons of users immediately. They’re betting that many of those users will slowly get used to the new site, or they can remove access to the old site in smaller groups instead of all at once. They also could be trying to work out bugs on the new site and the old one gives them a decent fallback for people. Also could be that some of the more obscure functionality of the site has not been ported to the new design, so they can link into the old one for those pages.