Even before that Abadi lacked the votes in parliament to get elected. On top of that Ajatollah Sistani let it be known that he will not support any candidate who had already tried and failed to solve Iraq's problems. Former Prime Minister Maliki and Abadi both fall into that category.
U.S. envoy Brett McGurk had tried to press the Sunni and Kurdish factions to support Abadi and encouraged the Saudis to bribe Moqtada al-Sadr. He failed. It now looks like the various Shia parties, plus some Sunni and Kurdish independents, have a solid coalition large enough to reign the country. Most of them want the U.S. military to leave Iraq. Iraq will get a new prime minister and it will not be the person that U.S. would like to see in that role. This will create some serious logistic difficulties for the increasing U.S. deployment in Syria.
The fight against ISIS in Iraq is not over. The political instability allows ISIS to return (vid) in the form of a lose guerrilla army. Iraq still lacks some tools, intelligence and nighttime fighting capabilities, to wear down these groups. With an anti-U.S. government the ISIS problem will certainly increase as the U.S. will gain use ISIS to keep a foot in the door.