Because I would guess that the Admins want to avoid (for example) a Bounty Hunter going out, killing an easy target, docking, renaming, doing it over again and again. (Though this could be circumvented, just not in the speed as an unlimited /renameme does)
To avoid abuse of the command.
As example, you were killed in a PvP fight, you rename yourself, reengage. Take the 5 million bounty. And you've still earned 3 millions after the rename.
Or you could just simply, avoid player interaction. As example ; someone sets out an individual bounty on you, so that other players can claim it and you not simply rename your ship.