Inspired by a twitter conversation with beanbunny
Most fantasy series don't capitalize nonhuman species (elf, dwarf, hobbit, etc.). Most science fiction series do (Klingon, Vulcan, Wookie, Ewok, etc.). My theory is that fantasy is working from myth and biology, where we don't usually capitalize other species (dryad, angel, crow, jackalope, etc.). Science fiction is working from something like nationalities, where we do capitalize different groups of people (American, Russian, Indian, Chinese, etc.).
Related idea: a lot of SFnal aliens have either planetary-wide cultures*, or two cultures, one of which is a Persecuted Minority. In that sense, the analogy makes a lot of sense: if you start going 'Space Soviets', 'Space Japanese', 'Space Jews'. Which has really Unfortunate Implications, now that I type it out.
Fantasy isn't immune to that, but fantasy also occasionally has a more narrow geography to work with. If you have only one group of elves who all live in the same woods, then it makes sense that all of them share a culture as much as all New Yorkers have things in common that even folks from New England don't get. When your Space Elves have forty planets and there's no regional variations between them... it starts to get weird. (Especially if humans still retain ethnic and regional identities.)
* If not one culture across all planets that a species owns. Maybe related to the whole 'one climate per planet' thing.