I think it's definitely taking longer races to sell out. I think the half distance is extremely daunting to non runners and serves as a barrier to many runners.
I am guessing there are a few other factors in play, as well. One, there have been a number of articles in the running world describing the burst of the marathon bubble: https://www.nytimes.com/2017/11/05/sports/ny-marathon-running.html
So, a decrease in number of participants but more half and marathon distance races than ever.
In addition, I had read in a few places about the substantial increase in slots at Disney races. Combine those two factors, and you have a slower fill-time (or no fill at all - and agree about the longer distances being more of a barrier) in recent years.