If the Trump sweetheart enigma birthday letter was the worst of what's in there, why wouldn't Trump let 'er rip and now make the Epstein files public?

Because that would be terrible for him, politically.

It's what I've been talking about in this thread. A sizable part of the MAGA base is powered by the belief that the government possesses conclusive and shocking evidence that the rich and powerful have regularly been engaged in pedophilia. That they could go out and arrest a bunch of these folks tomorrow, but that they are choosing not to because they are corruptly in thrall to the Powers That Be. This goes back well before Epstein - it was an animating force behind the QAnon movement. It's an eschatological belief framework. The government has been covering up this terrible abuse of kids for years, Trump will be elected and finally expose it, and all those horrible people who have been having sex with kids will be hauled off to jail.

That's what they were told. First by Q, then by a sizable industry of influencers, and then by the political operations of MAGA (especially after several of those influencers were elevated into positions within MAGA). And a sizable portion of his MAGA base firmly believes that the Establishment Powers were deeply involved in concealing sex abuse. It gave those Trump supporters enthusiasm and energy and commitment and a deep belief that their movement was an important factor in correcting one of the worst injustices and conspiracies in our country's history.

They will be exceedingly angry if Trump were to tell them that there's nothing there. The most likely response will be disbelief and to start thinking Trump's part of the cover-up: if Trump isn't marching the Evil Sex Abusers off to jail, then he must be part of the Evil Sex Abuser Protection Establishment. They can't have been wrong about the Establishment Politicians engaging in terrible sex abuse. That would mean that all their anger, all their sense of horrible injustice, all of the time and energy they devoted to all of this was wasted. That can't be correct.

But even for the ones that accept his explanation, it's still bad for him - because if they accept they were deceived about this, they'll become less enthusiastic about him and his political project. That part of the base doesn't strongly care about tax cuts and tariff rates and shutting down USAID or more stringent Medicaid access: they were excited because they were on the side of Good in the face of Unthinkable Evil. Take away the Unthinkable Evil....and what are they left with?