The end was announced in August, The Farm (which did not include Angela Kinsey) was not picked up in October, and Ed Helms was making Hangover III from September to November. Usually a long-running show can go out on its own terms but it seems the circumstances were dictated largely by the above.
Skimming the season summary
: "The subplot, however, involving a boom mic operator for the in-series documentary, was met with mostly negative reviews... later revealed in an interview that the subplot was a red herring to keep viewers emotionally invested in Pam and Jim's story."
"The triangle [between Andy, Erin, and Pete] has stirred up a number of debates in the writers room, which tells me even they don't know at this point which guy Erin will choose." It seems Ed Helms was a contingency plan for Steve Carell's eventual departure, but they really never knew what to do with him. Maybe Ellie Kemper's character's orphan background and attachment issues explains why a nice girl sleeps with half the office? They should have spent the final season shedding late-addition characters starting with those two instead of making them so unwatchable. I don't know how much they cost after Bridesmaids & Hangover, but it probably could have paid for a couple more episodes with likeable characters in watchable subplots.