He told SFX that studio politics at ABC were to blame for the demise of the series.
"[The cancellation] wasn't that much of a shock," he said. "I thought it would go on a little longer. There's a lot of stuff that went on with FlashForward that wasn't anything to do with the ratings. It was to do with [what was happening] inside the studio and... changes that were taking place."
However, Monaghan confessed that he had been disappointed by the decision to axe the show.
"I just thought it was really well-written," he said. "I played a very sweet, cuddly, nice character in Lost [so] I wanted to play a character who wasn't sympathetic. Not an easy character for an audience to like."
The actor added that he had enjoyed exploring the dark side of his character Simon Campos.
"It doesn't sound very nice, but I really liked doing the scene where I strangled a guy to death," he said. "That was really fun for me. I went somewhere a little different and enjoyed the final result."
The final episode of FlashForward aired on May 27 on ABC.