Obviously because her eyes are that color, but I suspect there are deeper, symbolic reasons for that particular color and description.
This is in regard to the epithet γλαυκῶπις, in the lexicon defined as "bright eyed", but which is often translated as "gray eyed" by poets.
The word γλαυκῶπις shares its root with γλαύξ, the word for owl.
Owls are known for their large and distinctive eyes, which are adapted for low light hunting. Thus, I believe the epithet is a comment on Athena's perceptiveness, telling us that the goddess of wisdom can see even through the dark.
You may find more information on Athena's association with owls in Wikipedia.
The currently preferred view of Greek etymologists is that γλαύξ "owl" and the colour-adjective γλαυκός (in Homer the colour of the sea, later generally “grey”) are not etymologically connected. If γλαυκῶπις is connected with the former, then it means "with the eyes of an owl", not "grey-eyed". The owl is Athena’s special animal; she is generally represented with an owl on her shoulder.
It depends, as Yannis said, on the translation. Wikipedia says:
In Homer's epic works, Athena's most common epithet is Glaukopis (γλαυκῶπις), which usually is translated as, "bright-eyed" or "with gleaming eyes". The word is a combination of glaukós (γλαυκός, meaning "gleaming, silvery", and later, "bluish-green" or "gray") and ṓps (ὤψ, "eye, face"). It is interesting to note that glaúx (γλαύξ, "little owl") is from the same root, presumably according to some, because of the bird's own distinctive eyes.
If the word for "grey-eyed" and "owl" share a linguistic history, then the correct translation would be "bright-eyed" since literally it would read, "with the eyes of an owl" which generally fall in the spectrum of yellow or a neighboring color. This would make sense being that the owl is almost always associated with Athene
Lately I have been reading terence McKenna. He thinks there is a connection between glaucous (meaning blue gray) and rituals. He suggests that there is a connection between the blue gray color of the musk (light film) on grapes. It's a blue gray color. He also suggests the glaucous color has connections with the color of a bruised mushroom. And, the more I learn about greek myth I read about eye opening hallucinogenics that could be the food of the gods. That people who visited Eleusis and participated the ritual there. were taking a combination of wine, mushrooms and other herbs. musky blue-gray, silvery gleam, bright eyed. I am sure there are many more interpolation attempts. I read about Glaucous, Deucalion, and now Athena. The more I read the more i find that greek rituals were about ever lasting life. birth, death, and rebirth. Seeing the real world, the real truth. The person becomes immoral.
Mythic authors didn't just use etymology, but used paronomasia, parallel naming employing names that are parallel by meaning, or by visual, or, phonetic structure. Athena is Grey-eyed because olives are grey. She symbolizes the olive harvest. Athena gave the olive tree to Athens. It is the year's last harvest, associating mature age with wisdom as ancients often did.
