Actually, he should have been banned years before he finally was. That event that got him banned was when he used social media to organize an attack on the federal government to stop the constitutional act of certifying the new government.
trump had two Twitter accounts: an official whitehouse account and his personal account. Trump’s own administration has stated that when Trump tweets from therealdonaldtrump account, he’s tweeting as a private citizen, so he’s not required by the same limitations of a government account (for example to promote Goya products). So Twitter banned the private citizen trump but not the official whitehouse trump.
if Obama invited people to storm the capitol I would be in favor of his being banned. if Obama cast doubts on an election he lost without any actual evidence I would be in favor of a ban as well.