Obviously, it's because he's working with the same personnel that played under Perry Fewell last season, except for Schobel (and he's not worth the massive drop off in performance), and the defense has been horrible. In fact they used 2 of their first 3 draft picks on defensive players. Putting your players in a position to be successful and adopting schemes to get the most out of them is the definition of good coaching. Playing defenses that you don't have the players to execute or asking players to play positions that they can't (see Kelsay) is bad coaching. What's even scarier is that he's tried to go back to something similar to what the Bills had last season and the performance is still awful. That's doesn't speak well for his ability to make good adjustments or game plan.
He still has the rest of the season to show something but he's not looking too good so far. I'm actually curious to see what Gailey does if the performance doesn't improve. It's one thing for a coach to can players that he didn't draft and has no commitment to, but another to can his hand selected DC after one season.