I also think the Star Wars stuff has simply been a lot more popular.
I'm not so sure I agree. In July I was still seeing last year's SW advent calendar in stores. Parents do not want to pay the price for that advent calendar. Other parents are offended by the inclusion of SW into an advent calendar in principle. Although, I"m not sure how it is different from a pirate, or a castle advent calendar...violence is still a theme. But that is off topic.
Advent calendars have been in the past, planned and numbers estimated years in advance. Remember when the US wasn't part of the plan for the first castle advent calendar? We just got lucky with an accidental shipment to the US?
I have been an avid advent calendar buyer since 2004 before they had minifigs in them. The best advent calendar ever was the Belville one. Don't believe me, research it. It was FULL of food parts. At the time food wasn't an easy thing to gather at a cheap price. This is the first year that I won't be buying one. The city is the same every year. SW is just not my thing. And the Friends one is lacking.
Will there be another castle advent calendar? Who knows. I wouldn't bet on it with SW and Friends getting ones...and City rules the LEGO world.