I think that with some varieties, there may only be 2 or 3 cuttings total available. Maybe that's why.
FWIW, I've not had good luck with UC Davis the past three years. I send in my request well before the deadline and either never receive an email from them or I do get an email but no cuttings. It's hard for folks like me to get a couple of new varieties here and there when you don't have big bucks for cuttings or to buy plants from nurseries. So it goes...