It's not that he's not releasing his tax returns, it's that his lawyers are telling the IRS not to comply with the demand of the house judicial committee to release Trumps tax returns, and the IRS is siding with Trumps lawyers rather than complying with congress.
There is a systemic problem with the Trump administrations - boot licking sycophants everywhere are willing to break the law to suck up their golden god, and Trump is on record encouraging it.