Well the imps themselves don't really have much of an interest in Althare (aside from collecting infernal artifacts), so conquering the whole thing wouldn't really matter much to them. They're all pretty eager to get back home to the Hells. Hek and Delilah would certainly have a vested interest in gaining more influence in Althare, though. So conquering the entire island isn't necessarily the goal, it would give you the best chance of becoming powerful enough to take on Beleth.
There'll be more lore drops to explain it, but Althare is technically unaffiliated with any of the larger kingdoms and lacks any kind of central government or authority. It's mostly populated by dangerous wildlife and a sleuth of independent factions (the Red Nettles being one such example). It has a lot of valuable resources, but most of the other kingdoms are doing too well off to justify any attempts to conquer it.
So while certain academic, mercantile, or mercenary groups would have an interest, most of the kingdoms wouldn't consider conquering and holding Althare to be worth the investment.