The amount of code (and bugs as a consequence) that will be introduced if Anet decided to allow a race change ability would require a huge amount of resource and time. All that time and labor could be better spent for more practical things right now, like the next expansion. I'm not against race changing, but doing so would obviously interfere with a lot of character-bound mechanics (progression, personal story, cultural armor, etc).
I just don't trust the Gw2 engine and the spaghetti code that much. It's incredibly unstable, especially when most patches always seem break the game in some very noticeable ways lol.