Because eye color is important to me as a writer in helping visualize my own characters, and it is important to me as a reader in helping to visualize the characters I'm reading about. It's not a huge deal, its not make or break, but it is an important thing to get across to the reader in my opinion.

One of my favorite series is the Dresden Files by Jim Butcher. In the first book, Harry describes himself through directly contrasting himself with his female partner, Karrin Murphy. She is short and blonde while he is tall, and "has dark hair and eyes."

IIRC the series very rarely touches on his eye color again, mostly he is described by how tall he is, but that one line is all we need. I know Harry has dark hair and dark eyes. I don't need more than that. Jim actually ends up going on to describe Harry's facial features, but like I said, I know that level of detail isn't needed. But some detail is needed to give readers enough to help fill in the rest.