Aye. It's geographical, I guess. In my region, across various points in time I've seen... Images and Words, Awake, Live at the Marquee, Falling Into Infinity, Scenes from a Memory, Live Scenes from New York, Six Degrees of Inner Turbulence, Live at Budokan, Score, Octavarium, Systematic Chaos, Chaos in Motion, Black Clouds & Silver Linings (multiple editions), and a Japanese import of Train of Thought, strangely enough. Also seen Dream and Day at a second hand shop, if that counts. All between my two hometowns, which are about a mile or two away from each other.
I'm not in a particularly huge population centre, nor a capital of... anything, really, nor are there any major gigs in the area. Maybe there's an appetite for it among posh teenagers, so they keep the shelves stocked? Hard to say. Heck, I've seen Dominici albums on the shelves, and they're not the best-known band even within the prog-world. (Sorry Chaz, on the off-chance you're reading this - if it helps I bought 'em!)
Chance is all it is, I guess. Ferret around a little more, though. And request 'em at the counter. Let them know there's a minor following. I have no idea whether that will work, having never worked behind the scenes at a music shop, but hey! Gotta be worth a shot.
Most places will get them bought in, too, if you ask. "Is there anything else you were looking for?" "Yeah, I want every single album by Dream Theater cheers." "Is that 'Every Single Album' with spaces?" "..." "Har har. Okay, we don't have any but we'll order it in for you. Come back in five days." "Lush."