I'm sure there is not enough demand to offer multiple calibers every year in all the variations. Looks like the plan is to offer one caliber in each of the variations per year. Not many people shooting single shots these days.
I'm sure you will see Distributer Exclusives on top of those, such as Lipseys does. That's practical for Ruger because they generally get a firm sale of 250 guns to a distributer, and let them take the risk of getting them sold.