Because it was an active investigation. If they released the investigation files, it would have fucked over the court case to convict them. It wasn't until recently that the Trump-controlled DoJ said "okay, we're done. Nothing to see here." Now that there's no longer an investigation, there's no longer a reason to withhold the files.

And before you say that they deliberately took too long to avoid releasing them, remember that the Democrats believed they had time to finish the investigation properly, but Trump's elections screwed that; and that when dealing with people with this much power, you need a case that is more than air tight, and that takes a lot of time given who is involved. Unless you believe the Democrats could have stalled though a second term (obviously Trump couldn't).

Oh, and if the Dems actually wanted to avoid this whole thing, they could have much more believably said "nope, nothing here," but instead they chose to push forward.

Did he mention "the files" ever? Campaign on releasing them?

Why are you taking for granted that there is some material thing that exists and constitutes "the files"? How is it not just yet another thing MAGA fabricated and paraded around as a boogeyman?