Previously theoretic risks are now manifest, demonstrated, and cognizable.
Actual riots, insurrection, disruption of government, assaults on law enforcement, damage to government buildings, theft of government property, violence, and deaths, have occurred.
The instigator is unrepentent and non-credible in what are at best tepid disavowals.
The totality of context has changed. If "he has tweeted worse things in the past", the undeniable impact is abundantly clear.
The ban is absolutely warranted. If anything, it is many years too late.
Twitter specifically address these points:
Due to the ongoing tensions in the United States, and an uptick in the global conversation in regards to the people who violently stormed the Capitol on January 6, 2021, these two Tweets must be read in the context of broader events in the country and the ways in which the President’s statements can be mobilized by different audiences, including to incite violence, as well as in the context of the pattern of behavior from this account in recent weeks. After assessing the language in these Tweets against our Glorification of Violence policy, we have determined that these Tweets are in violation of the Glorification of Violence Policy and the user @realDonaldTrump should be immediately permanently suspended from the service.
From TFA