Basically, the Z fares on this route have either a 14-day or 21-day advance purchase requirement and 2/14 is less than 14 days out. So the only fares available for purchase this close in are going to be I or higher even if there is Z bucket availability open. Domestic Z fares also have an underlying coach fare class, so there can be Z open and you can meet the advance purchase requirement for a Z fare, but the Z fare still can't be purchased because there is no inventory in the underlying coach bucket for the Z fare.