I can elaborate further if there is any desire for that, but here is a basic synopsis.
Manchin was very much liked by the majority of the state when he was a state senator and then as Secretary of State. He had lost a primary for Governor previously, but won in a landslide when the incumbent decided not to run again and Manchin had announced his running again. He was a very popular Governor for both terms.
When Robert C. Byrd died, Manchin appointed his young legal advisor as the interim and then announced that he would run for the open Senate spot. He won the special election.
Once in Washington, West Virginia began souring on him. This was mainly because West Virginia Democrats were becoming Republicans. It is a very long and winding road, but the basics are that southern West Virginians were 85% Democrats because John F. Kennedy’s father sent people with money down to secure JFK’s presidential election. Look up, “Don’t buy another vote; I won’t pay for a landslide.” But the majority of these people were not heavily educated and did not want progress or outsiders. They wanted to have the husband work in the coal mines; the wife in the kitchen; and a white couple next door.
Just like the Dixiecrats, these Democrats became Republicans and DINOs (Democrats In Name Only). They hated Manchin because he did not denounce the Democrats and become a full-blown Republican. (Carpetbagger, douchenozzle) Attorney General Patrick Morrisey ran against Manchin in the latest US Senate election and Manchin squeaked out a win. He will not win another term. This state is 1960s Mississippi right now, and Manchin is too moderate for them.
Long story summation, the Democrats did not want to vote for Joe Manchin but knew he was the only bastion of hope against having a rubber stamp Republican toadie in his spot.