Firstly, this should go to the "Pokémon Locations" thread. Anyway, I'll bother to answer this here.
GameFreak had always considered that bit of info since RSE. Notice where Slugmas are naturally found in the games. Slugmas are always found in hot/volcanic areas (Fiery Path, Route 113, Mt. Ember, & Stark Mountain). It is only that in GSC that they were put in an environment not suited to them (come on, Cycling Road?). Oh yeah, don't even think about putting them in the Brass (Burned) Tower, it doesn't make sense.
Funny that GSC introduced this Pokémon, yet they never gave it a proper and suitable location in those games. I just wish that somehow, in HGSS, that Slugmas be given a proper location in the wild. However, examining the HGSS Johto map, there are still no appropriate location for Slugmas since there are no hot/volcanic places in Johto/Kanto. So, I guess, we'll get to see Slugmas wandering off someplace which shortens its lives.
Sorry, wala tayong magagawa. Nandyan na yan eh...