1. It's possible that the original owner of that username was banned (although they should still show up on the friends list)
2. It's possible the username trips some sort of username profanity sensor.
I would bet on the second possibility, but I don't know for sure.
But since you have posted about this here in RSOF, the chance of you finally taking it is lesser (assuming the name is fine, and not offensive at all). When it comes to names, it's better not to mention it in a public forum. Availability of display names is a first-come, first serve basis.