Ultimately it's a political thing. It's not like a regular criminal case where you appeal to a higher authority and call on them to dispassionately judge the complaint on its merits.
Yes, that's part of it, but there's no higher authority than the federal government. Just the separation of highest authority in to separate, co-equal, and highly politicized branches.
Impeachment happens not when there is a crime to prosecute in the conventional sense, but when there is overwhelming public outcry demanding a political ouster. Public opinion on Trump, despite all his documented and alleged shenanigans, is still sharply split.
The Republican establishment would impeach in a heartbeat, if they thought it would energize Republican voters and increase their chances of picking up seats and staying in the Oval Office.
But Republican voters don't want Trump out of office that badly. And Democrats don't have the majority they need to override that public sentiment. So it's not going to happen.