It's not that he's required so much, its that he said he would.
In 2011 he said he would release his tax returns of Obama released his Birth Certificate. Obama released his 11 days later, but Trump did not. He also called on Mitt Romney to release his tax returns.
Today Trump is back peddling, saying he won't release his tax returns because he's under an IRS audit. There's nothing stopping him from releasing them because of an audit. Also recently he said he would only release them if Clinton released all of her emails, which actually, she already did months ago.
The latest from Trump is that now he is trying to bring Christianity into the mix.
https://www.washingtonpost.com/news/...ong-christian/