“Why does Trump want to withhold them? Seems they would reveal favorable things.”
That is naive thinking when you are dealing in the political realm.
Anti Trump partisans will only paint a negative narrative, while withholding evidence that supports Trump and Trump supporters.
Trump also doesn’t make it easy for them by fighting every one of their political maneuvers.
Do you not remember how they tried to spin the Ukraine transcript until Trump released it. But that still did not dissuade them. They tried to spin it into an impeachment, (which they lost), but the talking points were made so that the Rubes swallowed it hook, line and sinker. You still hear people claiming that Trump tried to get Ukraine to interfere with the 2020 election by investigating Biden and Hunter.