Because the trademark of Treasure Island is this made-up pirate language, it makes sense that Stevenson uses a lot of it. He wants to make the style of Treasure Island distinctive, so there's tons of dialogue in the book. Sure, Jim Hawkins has to narrate his adventures when he's alone (which he is for a fair number of chapters), but whenever there's another pirate on the scene, you can bet he's going to have the chance to speak, especially if that pirate is charismatic Long John Silver. That's why we say this book's style is both exotic and chatty: Stevenson is trying to create a new and compelling vision of pirates and how they talk, so of course he's going to give them as much space to chat with one another and with the good guys as possible. He needs to give us space to get used to the pirates' particular ways of speaking, because after all, it's the pirates that we're reading this novel for in the first place