Grazzt
Demon Lord
The reason is that I usually don't play with half-monster races, and I am tired of hearing "but it's in the Player's Handbook..." anytime I form a group to play in my local book store.
If you are the DM, simply say "no". Done. I've done it for years. No gnomes or halflings in my campaign (and haven't been since 1e). Likely never will be either.