The reason for the hats is to bolster traffic. With few exceptions policy for features depends on objective goals with a risk/reward analysis to make sure the feature doesn't do more harm than good.
The traffic dip is clearly seen in a visitor graph from Quantcast:
There's a million visit/day dip - an almost 25% drop in traffic - over this period of time, consistently, every year. Look at the bottom mini-graph for a clearer view of this year dip.
Further, the people who stop using Stack Overflow during this time period are not the people asking for help, they are the experts. Those who don't need Stack Overflow, but continue to visit and answer questions. When they are busy during the holidays, they simply stop coming, though they return after the holidays. So the balance shifts, and the people who need Stack Overflow continue to ask questions unabated, while those who answer them are reduced in numbers.
Stack Exchange isn't simply bolstering traffic, but they're trying to retain talent during this predictable, consistent dip.
The hats are a very clever way to encourage the community to stick around when other activities might pull them away. So while they are fun and entertaining, unless there is an actionable, necessary goal to be achieved during the Halloween season it is unlikely they would consider it.
If the summer dip (school is out, lots of people take vacations) was shorter, or more significant, you might have an argument for summer hats.