It could be a case of complexity with foreign keys etc, and maybe even older games or something architectural wise somewhere that relies on this (such as games grabbing this info). It's been a while since I've messed with databases though.
But just as an example as it has been brought up elsewhere when looking at this topic, UPlay allows you to change your name, but this wasn't always the case. If you play certain older games on UPlay, they will bring up your older name before you changed it as it seems to be the key and then another column for display name.
I guess they could do this for PSN as an alternative, especially as we now have real names on PS4, but they might not (and players will probably complain) if older names showed up in older games, which updating all of them is not really an option.