I really, honestly, having dog friends across many agility venues and many sports and play in several and can say pretty definitively:
Numbers are down across ALL agility venues, and markedly so. AKC and then USDAA seem the most stable but they're also having a notable falloff.
Numbers in flyball and obedience (not rally) are ALSO way the heck down. There's more stuff to do now and people are making decisions about what to do. More new people coming in, yeah, but not enough to compensate for the fact that it's no longer agility, obedience, or flyball. Now it's agility in one of 6 organizations, obedience with several organizations, rally with several organizations, dock diving with multiple organization, disc with multiple organizations, nosework with at least a couple of organizations, freestyle with at least a couple of organizations, herding with at least a couple of organizations, treibball, barn hunt, parkour (also with at least a couple of organizations) - and more organizations offering flyball and obedience and agility, too.
WHY ARE ENTRIES FALLING OFF? Some of those sports in various ways have been around forever, but they're spreading to new areas and new organizations are being made. How in the WORLD is it even possible to think that numbers in any individual trial isn't going to fall with that many options?
Yes, most people are going to go where their instructor recommends or is already at (if they're doing so rather than being professionally neutral) but I really, really don't think this is some mysterious thing. Probably a complicated one with many, many factors, but honestly? OPTIONS EVERYWHERE. For sports and for venue, for live and for videoed (rally, freestyle, and parkour all have online divisions/ability to title). It just - there aren't enough new people, time and money to see that kind of growth and not having entries into trials, venues, or sports drop off.