ARGGGHH. Something stuck in Print queue was the culprit for me. A print job had been in there for a little over a month since my wife used my MBP to print mailing labels for Christmas cards. But I only realized that my MBP wasn't sleeping 3 days ago. Thanks to this thread, I checked my Print Queue and there was a job stuck there! I deleted it, and my laptop is sleeping again with the power cord attached (it was sleeping if it was unattached.)
I found out I had an emulator running and happens with either iOS and Android emulators. Found this out by exiting all the apps one at a time and checking pmset -g assertions until there were no assetions left. Also got rid of any old printers that I did not recognize, but for me the real culprit was coreaudiod via the emulators.