Why did the Supreme Court vacate the ruling that Trump could not block Twitter users?

Question

In 2019, an appeals court upheld a decision that President Trump violated the First Amendment by blocking users from his Twitter account.

Two days ago, the Supreme Court declared the case "moot" and vacated the appeals court's ruling. An article gives the following reason:

The high court dismissed the case because Trump is not in office so there is no longer a live case or controversy.

I do not understand this reasoning. Why not leave the appeals court's ruling on the record, because it is always possible a future president will try to block users while communicating with a personal social media account? Now if the issue pops up again in the future, a president will be allowed to block users for multiple years while the issue gets re-litigated from scratch.

Answer 1

The case was appealed and taken by SCOTUS. Then the case became moot:

• Trump was banned from Twitter for life, his account permanently suspended. As the Twitter account that was the core of the controversy no longer exists, Trump can't block any accounts anymore. As Trump is disallowed from making a new account, it can't arise with the same fact pattern.
• Trump is no longer in office.

The SCOTUS may only weigh in on live and active cases of controversy. The case is neither live nor active because it was rendered moot in January. As a result, it had to be dismissed, or the decision to take the case overturned.

Speculating why they chose one over the other is moot, as there is no majority opinion given and the judges won't answer such questions.

Answer 2

The Supreme Court has a lot of things they can do in theory, but in reality, there are only two options the Court likely considered:

One was to simply dismiss the writ of certiorari as improvidently granted due to neither party being significantly affected by the outcome of the case, as they did in Public Employees’ Retirement System of Mississippi v. IndyMac MBS.

The second was to vacate the lower court ruling and remand with instructions to the lower court to dismiss the case. That is what they did in this case.

Why not leave the appeals court's ruling on the record, because it is always possible a future president will try to block users while communicating with a personal social media account? Now if the issue pops up again in the future, a president will wrongly be allowed to block users for multiple years while the issue gets re-litigated from scratch.

That was their other option. The most obvious explanation for why they didn't pick that option is that the Supreme Court explicitly wanted this result, likely because at least some of them don't agree with your characterization that they would "wrongly be allowed".

I suspect what's going on here is that there is nobody to challenge this ruling in an adversarial situation. So the Supreme Court doesn't want a piece of precedent that resulted only from the administration being changed and would prefer that if there's going to a precedent-setting case on this, that it be aggressively litigated.

So essentially, they didn't want this on the books because it wouldn't get appealed and they weren't convinced it was decided correctly.