I would say, that elves would make Cosmere books too similar to the normal fantasy book. Brandon takes a lot of care in making his own races and elves are kind of... hackneyed. Dragons though are more like you would take all those people elves dwarves e.t.c. and do your own, not humanoid races. Like if you look closer, every dragon is really different and unique. Some of them are a big, majestic creatures, who can talk and often are a part of a legends even in fantasy books and films. Another ones are just big monsters, who attack the world. Some of them are really big and some are only 4 or 5 meters long.