I can see at least one reason why. Follow my reasoning here. Yeast is a perishable product. There are two major suppliers of liquid yeast. Wyeast has 50 varieties of brewer's yeast, plus wine, mead, cider and sake yeasts. White labs, about the same.
So if a homebrew store wants to be THE local source for everyone who wants to ferment something, the owner must stock at least 100 strains of yeast. Now, I pay $7 for an XL smack pack. I don't know what the exact margin is, but a pretty standard retail markup is 100%, so for argument's sake let's say he paid $3.50 per pack. If he's smart, he'll stock multiples of the most popular strains, like 1056 and WLP001, so you can at least double the amount of units ordered, probably triple it.
That's a lot of inventory spoiling on your shelf.
If the locals aren't buying it - maybe they're using mostly dry yeasts or White Labs - it makes perfect sense to drop the line. If he doesn't he's going to throw good money after bad, letting smack pack after smack pack go out of date (spoil). That's loss. A good business doesn't tolerate loss.
Bob