Presidents divest their private businesses, usually putting them in a blind trust, in order to avoid conflicts of interest and breaking the Emoluments Clause of the Constitution. While blind trusts are not necessary to fulfill the requirements of the Emoluments Clause, Presidents Ronald Reagan, George H.W. Bush, Bill Clinton and George W. Bush all did so, while Obama placed his holdings into standard index funds.

Trump refused either a blind trust or relinquishing ownership of his company. He instead said he would resign from all positions and hand control to his sons, with the promise that they would not discuss anything business related.

In January 2017, Trump held his first press conference as President-Elect. He used a large stack of unlabeled folders as proof that he had handed over his company to his sons. He did not allow reporters to look at their contents.

Don and Eric are going to be running the company. They’re going to be running it in a very professional manner. They’re not going to discuss it with me. Again, I don’t have to do this.

Trump also called the emoluments clause of the Constitution “phony” and claimed that Article II of the Constitution gave him “the right to do whatever I want.” Trump also repeatedly promised to release his tax returns both during his campaign and after becoming President.

The Washington Post – Donald Trump won’t do what Ronald Reagan, George H.W. Bush, Bill Clinton and George W. Bush did
MarketWatch – Ethics laws don’t require Trump to give up control of his ‘unprecedented’ portfolio 
Reuters – Trump says won’t divest from his business while president
The Hill – Trump barred reporters from examining stacks of folders at press conference
Britannica – What Is the Emoluments Clause?