Without the Maritime Route, Saga City was just a remote place and its existence did not make any difference to Black Dragon Guild. Black Dragon Guild had already conquered five out of the six cities that belonged to Immortal Island. They were essentially known as the owners of Immortal Island.
Lava Teeth's sole purpose in Saga City was to fill up the empty seat of being the City Lord as well as to keep an eye on Karl who had the City Lord Seal. To describe his job in nastier terms, he was merely a warden.
Now that Saga City's authority fell into Sanguine Warflag's hands, Lava Teeth no longer had reasons to stay here.
The average ability of Sanguine Alliance players was way higher than Black Dragon Guild players and Sanguine Alliance had a comparable number of players as what Black Dragon Guild had in Saga City. If they were to really cross swords, Lava Teeth knew that they wouldn't be a match.