Because apparently, having lots of low-level elementals is still more important than thought-out abilities or high-level monsters that's not a dragon or outsider.
Anyway, these magical heat drills have an okay ability array, with a net +6 to stats. Medium size is pretty okay, high natural armor is very nice (but compensated for by the lack of wearable armor), and the fire subtype is probably a net gain. A burrow speed is okay, but hardly unattainable by other means.
3 elemental HD suck, though, as does the lack of limbs. Having only a single slam won't do your damage output any good, even if it comes with 2d6 fire damage and a burn effect attached. The (arguable) ability to charge without actually moving might come in handy for some builds, though.
+0 or -0? In the end, thoqquas are kind of lacking in the versatility department, being little more than a burrowing beatstick. Contrast the humble rogue, who has comparable damage output, much more useful skills, and better HD, and I think -0 is justificable. Do discuss, though.