Because the coaching, other than Def Coordinator, appeared to be horrible last season and an established head coach and Offensive Coordinator were brought in. Incognito was added to a poor offensive line. McCoy, Harvin & Clay were added as offensive weapons. It was presumed the defense would be just as good as they were last season and the offense would be improved.
Well, the offense has been significantly improved (9th in the league in points scored) but the defense has been much worse than last year. Roman may still frustrate fans with his playcalling but he's light years better than Hackett.
I believe most Bills fans expected an increase in wins over the 9-7 of last year and an end to the playoff drought. It can still happen but is unlikely.