I'm dubious as to what exactly Burton is bringing to the table here.
Well, stop motion, for one thing. Burton started out as an animator, and The Nightmare Before Christmas and Corpse Bride are certainly two of his most successful films (with the caveat the TNBC was produced and co-written by Burton but directed by Henry Selick).
And maybe that's the most important thing. The Addams characters started out in cartoon form, but they've only ever been animated in some toned-down Saturday morning cartoons from Hanna Barbera. As successful as the Sonnenfeld films were, they were still live action. An animated Addams Family feature could be truer to Addams's aesthetic -- and could perhaps go more authentically dark than the live-action movies could, since things that would be too creepy from a live actor might be acceptable as dark humor from a stop-motion puppet. (For instance, nobody wants to see Christopher Lloyd acting like the sexual predator the cartoons' Fester was implied to be, so he was given a more amiable characterization.)
I don't follow why he's doing all this adapted work anyway. He's a tremendously original creator whose strongest work has always been of his own invention.
That's where the money is. The studios want to make movies based on well-known properties because there's a built-in audience and a good chance of profit, and they want to maximize their chances by putting those projects in the hands of proven filmmakers who come with their own built-in audiences. So if a studio buys the rights to a property that's macabre, offbeat, and cartoony, the first thing they're going to do is call up Tim Burton's agent and offer to send him a few more truckloads of money if he'll do the film for them. He does have his own projects in development, but he's in demand for other, higher-profile things, because that's what happens to successful directors.