Until recently, he's claimed that since the IRS is auditing his previous years' tax returns, he can't release them, a lie because the IRS has stated that there's nothing preventing him from releasing his taxes, audit or no audit. He's not able to use that dodge with his 2016 tax returns, since they're not under audit.
What's in Rump's tax returns that he's so willing to buck overwhelming public opinion? Loans and investments from Russian oligarchs or Chinese banks? Continued financial connections with mobbed-up individuals or companies? Creative bookkeeping to hide his income? Likely yes, yes, and yes, and more. Whether he gives to charity (himself probably through self-dealing foundations) or not would only be a further affirmation of his narcissism, greed and lack of empathy, so that's not it. He's hiding something that could cripple his hold on power. That's why we need disclosure and why he won't give it.