tRump is a shyster. That seems obvious. He's spent his whole life trying to sell his properties, his lifestyle, and mostly his self-image like a used-car salesman. It's almost a wink-and-a-nod that everyone knows it's an act, but everyone plays along. He gets upset, however, when someone tries to spoil his party by bringing actual facts into the picture.

He's tried to maintain his "mystique" by hiding details, and threatening those who have them. He doesn't want his financials or his grades revealed, because then he can't control the narrative that he's "rich", a "good business man", and a "stable genius". The New York Times - doing some excellent research - has managed to do an end-around on tRump and release a report that shows how much money he lost over a 10 year period between 1985 and 1995.

Between those years (inclusive), he reported a net loss of anywhere from $4.5M to $918.5M, for a grand total of $1.17B in losses. Art of the Deal? More like Shart of the Deal. Because of those losses, he also didn't pay any taxes. From the article:

Because of this, he had to turn to other financing sources, such as Deutsche Bank and investors in Russia. Interestingly, at the same time, Russia was beginning to court tRump. From the book "The Road to Unfreedom" by Timothy Snyder:

There are 46 more examples at the link, although they come later in time, leading up to and occurring during the tRump campaign. However, it seems conveniently coincidental that Russia started cozying up to tRump around the same time that he was having financial trouble. It seems likely that's the REAL reason tRump doesn't want his financial details out there - they will jive just a little too squarely with the Mueller Report and other research such as Snyder's.