((Because I just wanted a pony with gray eyes. They were originally placeholder eyes until I decided her mother, but I kept them because they were pretty.
Besides, there are a number of theories as to how she received them. One is one of Cheese and Tomato’s paternal grandparents having gray eyes. Or having gray as a subset of blue (Flora, Coco, Rarity). Or deriving them from a coat or mane color (Cirrus, Maud). Or receiving them from one of her maternal grandparents (either having blue or gray eyes).
Pony genetics are kind of a funny thing, really. ))