Why have most countries withdrawn from the Union of South American Nations?

Question

Why did most countries withdraw from the Union of South American Nations? Is there a specific political reason for that? If they had joined prior to having withdrawn from the organization, then certainly something must have happened.

Answer 1

As noted in the comments, Wikipedia's UNASUR article lays it out pretty clearly. But let me try to highlight the particular bits that answer this specific question, it starts in the Leadership Crisis section...

After Colombian Ernesto Samper completed his term as Secretary General in January 2017, the UNASUR governments have been unable to reach consensus on the organization's future leadership. Venezuela, with the support of Bolivia and Suriname, has blocked the nomination of Argentine diplomat José Octavio Bordón.

In August 2017, six members of UNASUR ... joined in forming the Lima Group, a coalition of Western Hemisphere nations that repudiates the Venezuela government of Nicolás Maduro as antidemocratic. In February 2018, Maduro was disinvited from the Summit of the Americas hosted by Peru (held in Lima in April 2018).

They continue to cite specific reasons given by different leaders like Chilean Foreign Minister Roberto Ampuero saying things like...

"We can't be throwing this money to an institution that doesn't work."

Then various countries go on to fully renounce the treaty, despite Bolivia's attempts to keep it alive. I recommend reading the wikipedia article in full, despite the eventual dissolution it's interesting to see the group's goals and accomplishments.

Last but not least, here are a few other articles that more specifically discuss the withdrawals...

• Goodbye Unasur?
• UNASUR’s Dangerous Decline: The Risks...
• Chile begins formal proceedings to withdraw from Unasur
• Colombia's president Duque says will withdraw from Unasur bloc
• Ecuadorean congress confirms country's exit from weakened Unasur
• Uruguay withdraws from Unasur and suspends TIAR exit