Because the Bengals' offensive line was a hot mess this season. Just like Pollack paid for Tyron being hurt, Collins needing time to adjust to right tackle, the lack of a clear direction at left guard and the backup tackles playing awful, Anderson paid for not being able to coax his players to play better, even if it was more an issue of ability than coaching.
I think in most cases coaches get too much credit and blame. You draft three first-round linemen with the ability of Martin, Smith and Frederick, then your offensive line coach is going to look pretty good when all three of them are healthy and on the field together. If they stay healthy this year, Anderson has a chance to look like one of the better offensive line coaches in the league because Collins should improve as well and if we re-sign Cooper the line should have some cohesiveness.
Now, that's a simplistic approach to it because Anderson will likely bring some tweaks to the blocking scheme and techniques that most of us won't even notice. But it mostly comes down to the players. For example, none of the offensive line coaches he had could make Doug Free a great player. I think he did improve under Callahan, but if we had had a line full of Doug Frees, it wouldn't have been pretty because he was a serviceable starter at best.