There are two reasons I thought of:
1) A fully exotic ecosystem is both a huge burden on the world builder to create, and a huge burden on the players to understand. Many science fiction writers deliberately create worlds which they know are anachronistically too familiar accept in a small number of key elements in order to reduce the burden on the reader's understanding. The alternative would be to create a massive amount of exposition to explain the alien world to the reader. This would be detrimental to the story, and would impose a very high imaginitive burden on the writer.
2) Alien things and alien tropes don't really contribute to fantasy stories because that's not what fantasy is really about. Fantasy is about examining abstract concepts - particularly abstract normative concepts like good and evil - through the literary device of emboding these things as tangible entities. A 'non-bear' is only a useful device to a fantasy story if it stands for something that you want to talk about. Otherwise, it is just clutter. In this sense, the non-bear is probably less useful than bears precisely because bears at least already have been anthromorphicized and used to stand in for various abstract concepts that 'bearness' culturally and maybe instinctually arouse in us. We already 'know' the bear as the wise, affable, but slightly comic warrior-sage. It already has fantasy value. The 'non-bear' would have to work hard to obtain the same thing. Similarly, when golems and not-humans are used in fantasy, its rarely with the end of comparing them with humans so that we learn what it is to be human by contrasting with that which isn't. Rather, non-humans are usually stand ins for some philosophical idea - nature, violence, evil, or some bundle of these by appealing to a common cultural mythic narrative. Elves can stand in as symbols for alot of things, hense the fact that there are two sorts of fantasy settings: those that have them and those that consciously chose not to have them. To the extent that your setting abandoned this combination of familiar and ideas as tangible things, and went out its way to create truly alien things that weren't embodied ideas, it would feel more and more like science fiction (and would likely become recognizably science fiction at some point.)
Father of Dragons said:
I suppose it's because, to some extent, when everything is weird, nothing is weird. Which, come to think about it, might be an issue with Tékumel...
Good point. Wish I'd thought of that.
Tékumel has the problem for me of appearing at least from the outside to be about nothing. It's peices don't seem to have any purpose. While there may be no particular real reason why things would need to be familiar, there is a good literary reason why they should.