Well, the D&D / AD&D ones do anyway.
The reason: it's a chance to use all the B/X Dungeons & Dragons modules and the all Advanced Dungeons & Dragons modules that consume vast amounts of storage space without spending loads of time converting stuff.
That's basically it.
Except that's really not it, or at least, it doesn't explain everything. The more accurate reason: I've only actually run D&D twice. One adventure in grade school, when I didn't even know how RPGs were supposed to be run; and one campaign after college and working and back in the Philippines and using 3E rules and Forgotten Realms setting (which I really wish I'd been able to do more of, but that's another post), and neither of these was a proper dungeon crawl.
Ideally, I'd run a campaign in a primarily Mystara-inspired world (the setting for the B/X D&D game), because I liked several Gazetteers a lot: the Grand Duchy of Karameikos, the Principalities of Glantri, the Republic of Darokin, the Elves of Alfheim, the Dwarves of Rockhome, and the whole Hollow World idea. I also liked the X1 to X3 series of modules, and several of the B-series modules.
To round out the world, I'd slot some AD&D module locations into the overall map and history. The Saltmarsh series (U1 through U3), the Tomb of Horrors, the Temple of Elemental Evil series, etc.
Then, I'd tear out various portions of the map where the Gazetteer series kinds stumbled, or where non-Mystaran settings have far more interesting implications for the world at large. Farewell, Kingdom of Ierendi -- I think we'll try to shoehorn the City-State of the Invincible Overlord or Freeport into your slot. Good-bye, Five Shires, maybe we can transplant a Champion of Mystra-less version of the Dalelands into your area.