That was removed last Fall, during a period of particularly high account hackings, when ANet started using IGN as a security question.
As to why, I can not say for sure, but it seems reasonable that the Guru leadership felt it appropriate to remove that information which users might have left here, in public view, long before it became a security question.
If you wanted, you could just write 'IGN: blah blah' into one of the other fields. I think the reason for disabling it was so people didn't enter it because it was prompted at registration and not realising it was a possible security risk. There isn't a forum policy on whether you can put your IGN somewhere else if you wanted to.