It is also early March ... I'd guess the new to racing Cat 5 fields don't swell too much until May or June when the weather is nicer and more bike racing is on the TV.

edit:

some cat 5 fields from races I've recently attended:

crit on 3/8 had 40 in the cat 5 and there was a second cat 5 field run shortly after with about 20
Chico Stage Race cat 4/5 field had 60+
circuit race on 2/14 had 40 in the cat 5
crit on 2/13 had 50 in the cat 5
circuit race on 2/7 had about 20 in the cat 5

I'd think those numbers would be decent considering the wetter winter we've had.