Why would that be?
Declassification before the mid-terms could have had a significant impact, afterward, not so much.
I can see a tactical reason to do it afterwards. If the Dems retake the House the GOP can go on the media offensive. If the GOP retains both houses, then the GOP is that much stronger in pursuing justice in the case of FBI collusion with HRC/DNC/Russia et al.
The UK and Australian diplomatic cries should be ignored imo. But in delaying, Trump earns a “chip” to be traded in later. There is good reason I’m sure to want to work with our allies on this matter, for negotiating purposes on other matters no reason to inflame passions at the wrong time - but ultimately imo all of it should be declassified.
And declassification should be a good pretext on which the DOJ should pursue criminal charges against the conspirators. I cannot imagine Trump would let this slide forever. Payback must be delivered, and justice must be done for the sake of the country. Either with or without control of the House timing could be used to advantage; either as a counter-attack or in a strong position to move the ball many yards at once.