Because the reasons for it are tied to the renewed Sunwell and devotion to the light.
There are priest mages in the ranks of the Thalassians. Obviously not available for players for balance reasons, but that was a thing. And now, with the Sunwell containing holy energies in it, a mage who believes in the light and does so through the Sunwell is gonna have the golden eyes.
Well... except headcanon is one part of it, but canon explanation exist for non paladin or priest Blood elves with golden eyes, and it is because the thing is tied to the worshiping of the holy light through the Sunwell.
And the most important to remember here is that High elves and Blood elves don't worship the light the same way, and, by extent of that, they don't use the Sunwell the same way.
You see, Blood elves regained their faith to the light. However, the way they worship the light now has to do with the Sunwell, and they imbue with it instead of the classical way, and the classical way is the Church of the Light way, which Humans, Dwarves and High elves follow.
And a Blood elf could worship the light without the Sunwell, of course (Resulting in a light worshiper without golden eyes, given that it is due to the Sunwell). But the contrary doesn't happen. High elves don't abandon the Church in exchange for the Sunwell in order to worship the light. Doing so would be a betrayal not only to their past, but to their Alliance allies.
This is based on the way the light worshiping works. You don't simply change you source as if it was just another kind of magic. You have to be devoted to the light in order to receive blessings and powers from it. And also the Church is not only an spiritual institution, it is a political one affiliated with the Alliance. That happens to be the same place where High elves were and are.
I would need to find one more thing, and that is about Rommath, given he is one of the Sunwell's guardians. But the former text is more than enough to point at to why High elves and Blood elves don't follow the light the same way (as a whole) and don't use the Sunwell the same way.