I could give you an entire essay explaining it, but I have a life, so...
*Edit: I am just going to copy the post I made earlier this year.
I dislike Alicorn OCs for many reasons:
1: It takes almost no thought, you do not have to make the hard decision of what type of pony your OC will be, so you just use that stupid loophole and say it is every type.
2: It makes you look kind of full of yourself, giving the impression that you think you are superior to everypony else.
3: Alicorns are supposed to be EXTREMELY rare. So having one as your OC makes them more common and bland.
4: The 4 (5 including flurry heart) Alicorn princesses are the only known Alicorns. So you are going completely against cannon.
5: I REALLY WANT TO STRESS THE FIRST 2!!!!!!!!!
So yeah, that is why I hate Alicorn OCs.