There are three reasons, IMO -
1 ) Nothing will come of it, all it will do is appear to be a revenge tactic. That won't wash with those who think the Dems are all rainbows and light.
2 ) Once nothing comes of it, The President and his folks will declare they aren't guilty of the things the liberals painted them as, and all that will do is strengthen Republican numbers.
3 ) The President and the GOP do not operate in a vacuum. If this becomes a big issue, lots of things about Democrats are going to come out as well - things that the Rainbows and Light gang aren't going to want to hear in an election season.
In other words, Its in the Democrats best interest to not pursue this line, and instead let the imaginary 'truth' that is out on the internet about The President, Cheney, Rove, etc. continue to shape public opinion against Republicans. Just like NCLB -- If it was a smashing success, you damned well better believe it would be Ted Kennedy's brainchild. But since the Teachers Unions hate it -- guess who's fault it is ?