Before the election the Republican's held the house and were likely to hold it again. If it became public that there was FISA surveillance related to Republican campaign you would have to expect a House investigation. There was also a chance that Trump would be elected. You would assume if he was elected there would also be an executive investigation into the FISA surveillance which is what we see from the OIG and now the DOJ via Durham.
There are going to be very strong incentives for these investigations to try and dig up anything that would look like foul play. Even if you try and do everything by the book you risk burning what is presumably a useful investigative tool. I can't see how the next best option which would be try and get sealed warrants from a normal court is so much poorer that this loss would offset all the political risk.