Insurance! Many local policies are not 12-month policies. For example, my boat has to be winterized and inoperative from Dec. 1 - Mar. 31. I could pay for a winter rider, but once you get into late December through February, it's hit-or-miss with the weather. I Frostbite and when March comes around, I often wonder why I do it. December has some nice sailing, though!
Also, I don't know if this is unique to our area or not, but a lot of people that keep their boats on the Bay come from far elsewhere. For example, boats in my marina are owned by people that live in Texas and they fly in every now and then for some sailing. A LOT of people in Annapolis drive in from Philly and other parts of PA. Once it starts getting cold, the odds of making the trek to experience sucky weather get higher, so why not winterize and come back next spring?