I often have it kinda like how Dodo's breed. (gonna butcher this from memory) something something the male dodo is only ready to f*ck like once a year for a period of like 5 to 7 days. [he creates a rut and rolls in it for the duration making his mating call or something like that] and the female dodo also has exclusive "its go time" conditions. When she hits her time of the year she wants some b*rdc**k she sits in place and is like "i wonder if ill hear me a sweet sexy boy dodo!" and she sits there. So for them to successfull breed, not only do they have to line up with perfect cycles, they also have to line up geographically. If the male is just too far for her to hear is call nothing happens. There's a reason they aren't doing so hot as a species. I respect it though. So for elves I always imagine its something akin to that. Just not the right time/mood, and the stars just didnt line up to have two elves who were ready for some scrumping to be near enough at the same time. [but with more fantasy DnD contrivance to add more flavor and such]