Because Donald Trump remains completely uninterested in any aspect of his tantrum-fueled government shutdown that does not revolve around praise of or insults to himself, House Speaker Nancy Pelosi's decision to postpone his planned State of the Union address has stuck in his mind as no other aspect of the shutdown has. He barely mentions the 800,000 unpaid federal workers. He has rarely so much as touched glancingly on the damage wrought by closed government offices, or the steep daily drain on the economy that even his own administration admits continues to mount.
But the self-obsessed narcissist lost an evening of applause and television time, and he remains irate about that. After an initial attempt at retaliating by cancelling a Pelosi-led trip to visit troops in Afghanistan (his administration going so far as to expose not just the initial plans, but subsequent plans to attempt the trip on commercial airlines—yet another incident of Trump's team intentionally endangering the security of others in efforts to soothe the burger-chomping incompetent's ego), he's now musing about well what if he just shows up anyway, so there.
The idea of Donald Trump sniffing about the sanctity of contracts aside, Pelosi's deputy chief of staff Drew Hammill notes that there was no "written acceptance"; Trump is yet again lying. But the idea of Donald Trump storming to the Capitol to insist he be given his night of pomp and pageantry is a glorious one. Whatever else you might say about it, finding a way to make Trump's shutdown personally injure the tantrum-haver himself has been proven a masterful bit of gamesmanship. You can't get traction by showing a malignant narcissist the damage he's doing to others. A malignant narcissist only cares about damage done to themselves, personally.
And there is not a damn thing in life more important to Donald Trump than getting applauded by a crowd. Donald Trump would drop a nuclear bomb on any city in America you could name if he thought it would result in Donald Trump getting applauded by a crowd.
As to Trump's absolutely wonderful threat to just show up and give his speech anyway, there may be some problems with that. As in, it would appear to be logistically impossible, due to House rules and the fact that Donald Malignant Narcissist Trump is not in charge of Congress no matter how much the voices in his head insist otherwise. But it would be super to see him try. The man is seething, absolutely seething, and he’s not going to let this go. Ever.