Just read the lore. Is 
a fallen Angel or a Elf? Last time I checked, Angels have wings and halos but NO elf ears. Secondly, 
is Morgana's sister. Why doesn't SHE have elf ears or blue for that matter? These are the kind of questions that keep me up at night...
Actually, Shurelia (Ex Member of Riot and "Historian" in the lore 
) explained that the Pointed Ears where a sign of someone with a lot of Magical power... 
Janna/
Zilean has also this trait... and 
Ryze in some official pictures.
"The long ears are the sign of someone thats somewhat ascended/changed by magic. Janna has them too if you look." - Historian Shurelia, 21 March, 21 CLE
http://forums.na.leagueoflegends.com/board/showthread.php?t=596448&page=1#post7151136
(Of course that she is not talking about "THE Ascension" of Shurima)
Hold overs from their original designs, I imagine.
More specifically, pointed ears are not wholly the domain of Elves. Depending on which area you look, they are actually more connected with the daemonic order of things.
The original story was something about black magic having mutagenic effects, hence the wrecked wings and elongated ears. I think it was supposed to remind people of a fallen angel.
she simply has pointy ears,which isn't that uncommong among magical champs.
And pointy ears doesn't always equal with being an elf,just play any zelda game out there and lots of people have pointy ears,but aren't elves.
Why do and have Elf ears, while still be considered humans?
Why does lux have blonde hair? Why does udyr have a beard? Why is gragas covered in body paint? Why does any champion have any design?
