Because impeachment is for those who commit high crimes, misdemeanors, bribery or treason. That's why.
And interference with the election process isn't that? It's pretty clearly laid out that seeking help.from foreigners is against the law.
The concept that Trump was doing it for that reason is not only un-provable, but a lie by the commies. The phone call took place on July 25th. That was almost a year away from the Democrat party's decision on who their nominee would be.
In order to properly make that claim, the Democrats would have to do one or all of three things: Show us this crystal ball of theirs that predicted the win of Joe Biden; show us how they planned to rig the primary once again by putting Biden as the winner, and then prove that Trump was aware of these predictions.
Now, please show me this law that a US President seeking help from a foreign leader into providing evidence of wrongdoing by the past administration is unlawful.