For the same reason the Senate Dems did exactly that when the House impeached Clinton.
The Senate acts a jury and judge; they do not “try the facts.” That is for the House to do. And what do we have here? Lots of supposedly learned opinions and not a single fact to support the claims in the articles.
The House’s process was entirely partisan, so McConnel has hone on record as saying the Senate’s process will reflect the House’s.
Get over it.