Why? Because fuck you, that's why!
And now on to the Trump appointed courts. Good luck!

“In reliance on the advice of the Department of justice, we have determined that the Committee’s request lacks a legitimate legislative purpose, and pursuant to [federal tax code] section 6103, the Department is therefore not authorized to disclose the requested returns and return information,” Mnuchin wrote in a letter to Neal on Friday.

“For the same reasons, we are unable to provide the requested information in response to the Committee’s subpoena.”