There been a slight shortage since the early summer due to panic buying of the election. People are still buying wondering if the U.S. will sign the United Nations ban and firearms.
Next reason is that a lot of people who couldn't afford the price of an AR, now know they are not that expensive and are buying more and a lot of people are building them. Don't go by Dicks if you don't have to, it seems they either have it or don't no matter what time of year it is.