There's a million reasons

By definition a social network tries attract a large user base, controls a feed algorithm, ostensibly has SOME information on its users. Social medias sites typically also need SOME moderation, gov't moderating speech would be a nightmare in the US. And you throw in the weirdness of government being in the loop in our social lives. We also already have precedent on NSA, etc and not completely unfounded paranoia on government tracking its citizens.

So you not only would have the perception of government controlling speech, including worries about whatever administration is in control managing that speech, you have the boogeyman of social media and gov't tracking in one centralized place, gov't "in our private lives", and a million things anathema to American culture.

Mastodon instance, mostly for official communication with the fediverse seems fine though.