“The numbers show that in 1985, Mr. Trump reported losses of $46.1 million from his core businesses — largely casinos, hotels and retail space in apartment buildings. They continued to lose money every year, totaling $1.17 billion in losses for the decade.”
From: nytimes.com
I’m not a lawyer and do not have any special information here but one thing to keep in mind, a billion in declared losses may not be a billion in actual losses. He may be overstating or just inventing losses as a way to conceal profits. That would be tax fraud. And a 30 year history of felony tax fraud seems like a really good reason to block anyone from being able to look at his taxes. If they should become public, it would open him up possible fines and penalties, perhaps exceeding his ability to pay.
https://www.nytimes.com/interactive/2019/05/07/us/politics/donald-trump-taxes.html