Dale,
A few reasons.
1) They only sell what they know and/or can get large quantities of.
2) They only sell retail the varieties that are wanted at the wholesale level. If you sell 10,000 trees for a commercial planting, you "make" extras so you will have the full; 10,000. Then you have to sell what is left over.
3) It is easier to stock 5 varieties than 200.
4) They produce what has sold in the past (often because it was all that was available for people to buy because it was all that they sold- so it is a self-fulfilling prophecy, sort of).
Check almost any nursery catalog for any type of plant: cherries, apples, asparagus, whatever, and they all have a few varieties of whatever plant they sell.
on the flip side, often the varieties that are sold have proven, over time, to be good varieties, If you have a good experience with their product, you will likely come back and order again. So they sell what is safe.