Near extinction, the Kobliad needed deuridium to stabilize their cell structure, which prolonged their livespans. During the late 2360s, the Federation was working to supply them with the substance, but even despite new deposits opening up in the Gamma Quadrant after the discovery of the Bajoran wormhole, the substance remained too scarce to service their entire population. As a consequence, some Kobliad had gone underground to get it.
Background information Edit
The producers wanted just a minor set of makeup appliances for the appearance of the Kobliad in "The Passenger". As such, the makeup department created a simple ridged forehead that ran from the hairlne down to the tip of the actor's nose and a forehead piece with an extended set of ridges. (Star Trek: Aliens & Artifacts, p.150)
According to Star Trek: Star Charts (p. 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), there was a star system named Kobliad, which might possibly be the system of origin for this species, in the Alpha Quadrant. This was a binary system with a pair of F-class stars. In the late 24th century, it was an independent system.
In the Star Trek: Titan novels The Red King and Orion's Hounds, a Kobliad named Fo Hachesa serves aboard the USS Titan. The species is described in the books as sometimes being difficult to understand.