Despite contributing a Halloween outing post this year, I agree that they seem less prevalent than they once were.
Here's my data, from the Sandy spreadsheet...

From 1998 to 2013, I dressed in public on 15 days, 5 of which were Halloweens.

Since then, I've presented as female on Halloween 3 times, out of 86 Sandy-in-public days.

As dressing events become more frequent, there's less pressure to "go big" on Halloween, I suppose.