Whats up with this username thing??

Why CANT I CHANGE MY USERNAME!!!!!!I love snap chat and I have very high steaks so I dont wanna make a new account and not have my memories on them with also a new email,THIS IS RIDICULOUS!!!!I hate my username and I have grown out of it so PLEASE let me change please please please just once at least!!!!!!!

Doctor who 920 about Snapchat, v10.15.0.0