Centralization reduces friction. Normies who sign up on Mastodon are going to want to be able to talk to all of the other Twitter refugees too. By making mastodon.social the default, it encourages centralization of the mainstream portion of Mastodon’s userbase, such as journalists, official company accounts, public figures, etc.
But most of them are probably going to use BlueSky. I heard journalists have been mostly gravitating towards that option.