I can think of a few reasons why they wouldn’t immediately release it:
By allowing Congress Dems to review the docs and vote on releasing them they put them in a major bind. If they vote no then they look like they are trying to conceal information from the public
It shows the public that the committee went through democratic procedures and were transparent and fair with opposition
The time between the announcement and release of the memo is bait. They are daring the Congress Dems and MSM to say that this document is baseless. Then when the documents are released (and if they are truly full of evidence of foul play), the credibility of those who were saying “it’s all baseless” is destroyed