I think it's because Harry wasn't thinking about them much. The typical Harry Dresden book is about one weekend and mentions every one he happens to interact with during that limited time frame. Twelve Months is about Harry spending a year to start moving on from a very traumatic battle while fully accepting his current place in life. Lacunna, Georgia, Butter's girlfriends, and a bunch of other minor characters weren't relevant to that story so they weren't included. I hope Jim will mention them soon but I'm not surprised that they aren't mentioned at all.