Because you can't remove someone's constitutional rights without
due process. Regardless of that, he'd been investigated twice by the FBI, and cleared. He was even a registered security guard.
One of the gun shops he tried to buy from called the FBI in regards to the sale due to him acting dodgy and the FBI decided not to pursue it!
The Democrats decided to vote against both pieces of legislation the Republicans brought to Congress to do something about firearms being purchased by people on the 'no fly list' today fyi.
I liked Colion Noir's response