Absolute Anime

OR options 6&7
6:it took to long to draw
7:he got contacts

Okay, here's a less "fun" and imaginative explanation:

Yoshihiro Togashi changed his mind at the beginning and decided to make Hiei a main character, and he came to the conclusion that not as many people would like him if he were covered in eyes.

Anomura wrote:Sensei also gave us some information on the mythology behind the manga. Apparently Hiei's green, many-eyed form is based off (I think he said Hindu?) mythology, on the figures of women who could not be with their lovers. We discussed the possibility of Togashi dropping hints that his editor had refused to let him include a shonen ai couple in the main cast of a shonen manga.

I wrote this in the "Hiei and Kurama's relationship is..." thread. That was my History of Manga sensei's explanation. I personally think it was Hiei's way of demonstrating control of himself, though. ^_^

ObsidianSamurai wrote:Actually, I believe it is because of the vulnerability.

It isn't smart to fight using a form that has big honking eyes all over. The eyes are just begging to be stabbed.

The eyes on the rest of his body just amplify his power. He seems to have no problem wearing pants over them, while the one of his forehead is vulnerable enough to lose power after being blinded.

Also note that he keeps it open while he fights. He may not transform, but..
That and he'd only seen Yusuke fight once before he admits high regards to him after his defeat of Seiryu, demonstrating that Hiei considers Yusuke a worthy opponent and has as early as their first fight