"Flynn first declined to comply with a Senate subpoena in May, asserting his Fifth Amendment rights. More recently, the committee has reiterated its request and Flynn has declined again, the source said.

Flynn has also been subpoenaed by the House intelligence committee, which is conducting a separate probe into Russia's election meddling. Flynn had offered to testify before both the Senate and House intelligence committees in exchange for immunity, but neither committee accepted the offer."

It appears he wants a get out of jail free card. If he did something wrong/illegal, I'm sure Americans want him help accountable. At least that's what we normally say about law breakers.