I wonder if they think certain settings are more likely to be treated as a one off lark than a long campaign. Dragonlance got a module, for example, much like Ravenloft did with Curse of Strahd. But Eberron got a full book, rather than the mixed bag. I don't own any of the MtG settings but I gather that Ravnica and There's at least were more traditional setting books. That both Spelljammer and Planescape got the hybrid product suggests that WotC maybe thinks those settings need a more concrete introduction with a built in adventure. If they ever were to do Dark Sun, I would expect similar treatment for similar reasons.