It could be a set up by the DNC. After rigging the primary it sounds like something they might do. Why didn't it come out before the election? Because the DNC realized Trump Jr. had done nothing wrong. Even if he intended to use info from the Kremlin, since none existed he did nothing illegal. You can't be arrested for intending to rob a bank if you never do. After the election they needed to throw fuel on the sputtering collusion fire and knew their mindless snowflake minions would swallow it up.
They did not rig the primaries. Democrat voters got who they wanted even if it was a questionable decision. If they went through all the trouble of setting it up then why wouldn't they use it? Trump Jr did something wrong. The court of public opinion is different from the court of law. Voters are already negative on Trump's relationship with Russia. The fact is that it does prove collusion or the willingness to collude. If you agree to a meeting to receive stolen goods, you are guilty even if you never get the stolen goods.