'Cuz 125 grain heads have got cooties.[8D]
Only downside I could see to all this would be that the 125 grain point might require the next stiffer spine rating, and subsquently move you into an arrow shaft that is heavier and/or fatter. My 61 lb. bow, for instance,can use the3-39 series arrows with 100 grain points; but actually needs the 3-49's for 125 grain heads.
Being right on the top edge of a shaft's spine rating and not wanting to buy more arrows is a good reason to go with 100's over 125's. Or maybe a preferred broadhead only comes in 100 grains and not in 125's. Other than that, I doubt anyone could give you a semi-coherent reason.