Despite the similar "D&D can be used for anything" trope, it didn't have any more real diversity than Traveller does. Where's the "Wild West" with D&D rules? Where's the 1950's Milwaukee Noir Urban Fantasy campaign with D&D rules? With D20, much of those splinters rose up. With GURPs those kinds of things rose up. Settings without mechanics. But pre-D20? With D&D? No.
My copy of the Dungeon Master Guide for AD&D has rules and data for the Wild West for those who wanted to mix genres a bit. And then there was that memorable Sturmgeschutz and Sorcery game. In the mid-1980s, when GenCon was at the University of Wisconsin-Parkside, there were some quite creative mixtures of D&D and AD&D with other games. My set of Little Tan Books, dating from circa 1977, with all supplements, includes encounter tables for Mars/Barsoom, as in John Carter of Mars, while Blackmoor has the Temple of the Frog adventure with an alien running the Temple with an emergency set of Battle Armor available.
I had a lot of fun with my players mixing the monsters of the various editions of the game. One player who ran a cleric freaked out the first time the group ran into Bone Golems. Just missed removing his head with one. It did keep them on their toes though, as they could no longer assume that they knew what was coming at them.
The last sentence sums up the problem. The guys in the Sturmgeschutz game had no idea that they were entering the D&D universe, while the D&D guys were WW2 Miniature players as well, and had no problems adapting. When you put players used to one milieu into another one that looks the same but works a bit differently, they can get a tad upset. Once adventures started coming out, clarifying what was meant by the "Imperium", and filling in more and more of the blanks, then players used to that type of game are going to be more and more reluctant to in effect "jump ship" for another different universe. If someone told me that the next gaming session was going to be in the Zhodani area, or the Aslan or Vargr areas, I would pass immediately.
I am setting up a sector were personal energy weapons do not exist, and civilians do not have Battle Armor, or anything better than Cloth (note, Cloth with my house rules), with small ships (5,000 or 10,000 Traveller dTon limit), and very limited psionics. That is not going to appeal to a wide range of players, as it is quite different from what the Official Traveller Universe, in all of its various permutations, is.
The key to the wider options for differing milieu is the ability to sell downloadable books online. If you wanted to publish a differing setting for Traveller in the early 1980s, you were looking at hard copy printing, which did represent considerable up front costs. Now, with the Internet you can avoid the up front costs of printing, and for the hard copy people, you have the Print-On-Demand option. That changes the cost equation drastically, and makes something like
Attack Squadron: Roswell, by Zozer Games, a viable alternative setting for Mongoose Traveller players. (I liked the scenario so much, I bought Mongoose Traveller to use it.)
