I'd guess that it's because there isn;t much demand for it in your area, or that the store doesn;t think there is much demand, in which case you could enlighten them.
They're hard to eat, they don't curl around your fork like spaghetti and tend to fling the sauce on your clean shirt if you're not careful! Maybe that's why people don;t ask for them.
I don;t think they;re harder to make, they're just made of the same dough, extruded from a - what do you call it - a die? - with a pin in the middle (somehow, who knows how) like penne and other pasta with holes in the middle are. Pasta dough is pretty strong, since it's got a lot of gluten. Here they are not at all rare. I'd say it;s a question of lower demand or perception of demand.
Here, instead, I can't for the life of me find acini di pepe pasta. I love it in soup - it;s chewy but is also small. I hate the small pasta with the hole in it, because it has no bite. But they just don;t have it. It's extremely rare. I grew up on it in the states, so i'm imprinted on the feel and texture of acini di pepe. It does exist, but have found it very rarely. Perhaps a regional preference.