are you looking at the older categories? There just probably aren't that many 70 year old men willing to go to Madison in Jan to race cross. I also think numbers will be down because it's in Madison and there are worlds coming up in Louisville anyway. Just my 2cents.
As Beth said, location partially responsible - many feared brutal cold. Worlds partially responsible as others can't/won't travel internationally to attend them but prime opportunity this year with first Worlds in USA. And finally, you are correct. Many non-elite riders enter. If you look at the geography of the registrants, it's mostly mid-West this year and follows the same patterns when Nats are east/west coast. It's just difficult to fly a bike or two, take a week off of work, etc. etc. for a 45 minute race for many. It's an amateur sport and the time/expense are too much for every regional strong guy to attend. Masters Worlds fields this year are 99% USA with a few Canadians so the Euro guys apparently have the same issues.
I entered Nats 45 - 49 a few years ago as a cat IV just for the experience and to accompany a teammate. Perseverance in bad weather/tough course where many DNF'd and I finished just behind the arithmetical middle of the pack. However, I was definitely a poseur in that setting - the winner (Tilford) put two laps on me (out of 6). As the sport has grown, the fields are more full of talent and I wouldn't feel right entering Nats at this point even though I'm a better rider. Next time they're here in the East, I'll enter the 'B' non-championship field. Also, as I hit 60 and up (not far away for me), I would have no qualms entering those smaller fields. However, given the growing popularity of the sport, I suspect that even the grandpa and great-grandpa fields will be more full as we late-Boomers continue racing. If one is finishing in the top 1/3 of a large regional series (MAC, MABRA, etc) in your age, reasonable to enter Nats/Worlds in the large fields IMHO. Otherwise, someone's just another course obstacle for the contenders at the start and in the late laps. Just my $0.02 from the back of the pack.
a stellar reply, thank you.
i wasn't aware they had non-champ races, it would be fun and a challenge to ride the course. that would better suited. being in the way when lapped is not my goal.