The Democrats didn't want the impeachment proceedings to bleed into the general election campaign and so they moved forward without trying to compel witness testimony through the federal court system. It is a political process, with political considerations being made. It's true that Democrats would not allow Republicans to call witnesses such as Alexandra Chalupa, Adam Schiff and the Bidens, but those witnesses were not germane to the core issue of whether or not Donald Trump withheld military aid to the Ukraine in exchange for a campaign favor. The Republicans were trying to deflect and shift the focus away from Trump. The Democrats didn't allow it. I suppose it's reasonable to call that "unfair" because in an actual trial, the defense is allowed to deflect and shift the focus away from the defendant. It's what a good defense team tries to do. The Democrats were in a position to keep that from happening, and so they did.
Is this executive privilege we are talking about, or entitlement? The White House's refusal to comply with the essential function of oversight will set a bad precedent for future administrations to follow... and could come back to bite Republicans one day. Eventually, there will be a President from the Democratic Party again, with a Republican-controlled House of Representatives. Good luck ever getting a subpoena complied with when that day comes.