Because Robert Mueller, the special prosecutor on this case, is not done with this investigation. This turned into a Criminal Investigation just a little over a month ago on May 17, 2017. He is in charge now--he has plenty of evidence--and I believe he is right now looking at bank accounts--investments, he is also interviewing people like Paul Manafort, and those that can verifty and testify to James Comey's memo's etc.
What I do know is Mueller has hired a criminal law expert to his team.
Robert Mueller staffing up Russia probe - CNNPolitics.com
We aren't going to know much more until Mueller has completed his investigation, then he will brief the Senate and House Intelligence committees--and it will be up to them to proceed or not proceed with impeachment proceedings.
In my mind and from what I have seen over these last several months, Republicans have two options.
1. Remove Trump from office asap or
2. Burn the entire party down in 2018--2020 & beyond.
If Republicans can do 8 investigations into Benghazi, it's not hard to imagine what Democrats would do with Treason, Obstruction & Lies when they take over in 2018. You'll also get a great education on what that Emoluments clause in the Constitution is all about.
https://www.brookings.edu/research/...xt-meaning-and-application-to-donald-j-trump/