DandD said:
One has one of the most famous miniature skirmish battlegame-lines, whereas the other doesn't, I guess, which is also the reason why so many people know it in the first place.
Also, according to Wikipedia, that Tekumel-setting reads like some kind of scifi-stuff set to become a medieval fantasy-world. Well, you shouldn't mix scifi and fantasy too much, I'd say, and Games Workshop realized that, which is why they officially separated the Warhammer Fantasy World from the Warhammer 40.000-Universe and let both franchise stand on its own.
Of course, Wikipedia does list that another factor contributing to the decline of popularity of Tekumel lies with TSR's lacking support.
Well, as always, I guess that there's more than meets the eyes.
Okay, so maybe these weren't the best examples. But many of the most popular settings draw upon things with which the audience is strongly familiar.
World of Darkness: Modern-day Earth plus supernatural entities.
Eberron: Pulp adventures (plus that "Roaring Twenties" feel for those with an interest in history).
Call of Cthulhu: Our Earth at various time periods, plus entities of cosmic horror.
Forgotten Realms: Lots of similarities to Tolkien and real world history (though it seems those are getting deemphasized in 4E).
Shadowrun: Near-future Earth with cyperpunk technology and many of the most popular fantasy tropes (again, lots of Tolkien influence here).
Exalted: Lots of anime and wuxia influences.
On the other hand, some rather creative settings were not exactly commercial successes, even apart from the above-mentioned Tekumel:
Planescape: While based on standard (at the time) D&D cosmology, it diverged very far from the common fantasy tropes.
Spelljammer: Again, too wildly divergent from standard fantasy to gain mass appeal (in my opinion, they should have emphasized parallels to the historical Age of Sai much stronger - including a "default setting" with multiple "core worlds" similar in culture to the powers of Europe and a "colonial rim" similar to overseas colonies of the former.)
Dark Sun: Again, very different from any tropes and stories most people are familiar with.
Those settings all had plenty of originality, but that alone wasn't enough to make them a continued commercial success - despite the fact that they had the marketing of the largest game company out there behind them and a large number of supplements. It should be noted that the last of the "lost settings" of the AD&D2E time - Ravenloft - fared good enough to be picked up by another company for a time. The reason for this is, I suspect, that Ravenloft is based on a rather more familiar source than those other settings - i.e. gothic horror - and thus was more accessible than the rest.