The offical answer would be "It depends on the DM, ask your DM directly"
Some likely guesses may be:
-In the DM's idea of the world, non-standard races don't fit.
-Monsters as PC's might fit, but have a hard time walking into humanoid villages, cities, etc without being chased by an angry mob who doesn't understand their specialness.
-The DM doesn't want to deal with the added complication of mentally keeping track of such a different race of creature.
-The other players will be jelious of your awesomeness. And your PC will be stronger then theirs at first level.
-Mundane gear is generally built to standard bodies, and your PC will have a harder time getting stuff that fits.
-you, the player, have managed to annoy the DM in some other way, for whatever reason, and your DM refusing you this request is because he's irritated.
-Your DM was in a bad mood for reasons other than you, and refusing you is because he's irritated.
-Your bribe isn't big enough.
-Your idea might be accepted if you presented it in a slightly different way (Have an IRL higher Diplomacy score)
...really... I dunno.
As Dandu offered, if you do want to make something powerful that is more subtle that your DM is more likely to accept, we could offer suggestions. What exactly do you want?
I'm a fan of a Half-Minotaur templated Changeling Barbarian who goes Warshaper for the extra reach and the Fast Healing myself. With a Spiked Chain.