I think a lot of it has to do with marketing. That whole Chili's Baby Back Ribs campaign made them insanely popular, and there's something about "baby" as a term that seems to make menu items more popular. "Baby" carrots, "baby" arugula, "baby" swiss...add "baby" to something, it sounds cute, petite, more tender, whatever, and more people will buy it.
In my experience, prepared in the same way, baby back ribs might be a little bit more tender than spares, but I've never noticed much difference in flavor. Spares are generally meatier and usually about 2/3 the price of baby backs, and me being a cheap bastid, that's what I typically go for. Actually, the local warehouse store (BJs) recently started carrying "Pork Brisket Bones" for $1.99/lb. These appear to be the trimmings from full spares to make SLC, which I've cooked a bunch of times (any time I get full spares, actually; I almost always trim them to SLC), so I'm planning to pick some up soon.