Bloomberg reports:

Scheindlin in August granted a motion from Bout’s lawyers to exclude statements he made to DEA agents after his arrest in Bangkok in March 2008, saying the agents ignored Bout’s request for more time to decide whether to talk.

That was hardly the only reason. From the judge's order, available on PACER:

In sum, based on the totality of the circumstances just described – the dramatic arrest, followed by a strip search and a non-consensual search of his belongings; handcuffs and a perp walk; the denial of his request for an attorney and contact with his embassy; the disregard of his statements that he did not wish to meet with the Americans and then that he was not in the frame of mind to speak with them that day; and Bout’s understandable belief that he would be abandoned to the rough conditions of a Thai jail if he did not cooperate with the Americans – I conclude that Bout’s statements were not made voluntarily and must therefore be suppressed.

Bout's defense, via the BBC:

Viktor Bout's lawyer says his client was the victim of a bait-and-switch con scheme by ex-criminal US informants posing as Colombian rebels. The wire-tapped conversations between Viktor Bout and Carlos and Richard, the US Drug Enforcement Agency sources, will be at the heart of the case.

In opening argument:

His defence lawyer, Albert Dayan, argued that his client had only agreed with DEA operatives to secure the sale of two transport jets, for a total cost of $5m. He told the jury Mr Bout had subsequently lost his transport business and was working in real estate when he became the target of the US government sting.

"Viktor was baiting them along with the promise of arms, hoping just to sell his planes," Mr Dayan said.