Treasure Island is an adventure story. If there's one thing that Indiana Jones, that dude from Avatar, and Jim Hawkins all seem to agree on, it's that if you're looking for adventure, you have to go to unfamiliar places and meet new people. It's hard to have a real adventure in your own kitchen, for example. So Robert Louis Stevenson tries to create a deliberately alien and exotic effect in his descriptions of Treasure Island to make it seem more adventure-y. Whenever Long John Silver, Israel Hands, or even veteran seaman Captain Smollett speaks, their language is so suggestive of high seas life that we can almost taste the salt.
Take, for example, Long John Silver's dire warning to poor Jim Hawkins in Chapter 28: "Now, look you here, Jim Hawkins [...] you're within half a plank of death, and, what's a long sight worse, of torture" (28.44). His way of speaking is deliberately old-timey ("look you here," instead of "look here," or just "look") and full of pirate-like expressions ("within half a plank of death" is a reference to walking the plank). And the content of the sentence is also pretty terrifying – Long John Silver is basically saying, look, kid, you're about to get murdered, or even worse, tortured. His unusual way of expressing himself makes his threats seem all the more believable and scary. And it's not just Long John Silver – check out Billy Bones or Israel Hands for more amazingly salty language.
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.