Specialty coffee is a seasonal product. Like any other fruit, there will be different times of year that different regions are harvesting fresh crop. This is the main reason Cartel won't carry coffees for more than three or four months at a time.
There are three main variables when it comes to seasonality: Coffee grown north of the equator, south of the equator, and on the equator. North and south will have opposite seasonality for harvests while equatorial regions like those in Colombia will actually have two yearly harvests.