why cannot change username?


You can only change your username one time.
https://support.chess.com/customer/en/portal/articles/1444791-how-can-i-change-my-username-
Sometimes staff will reset it for you after that but you will need to open a ticket.
https://support.chess.com/customer/portal/emails/new

I know a member of this site who is something of an "eccentric".I am not going to name that person but it is clear although they have maintained the same account they have been able to change their username at least 5 times.In the light of what MS says in the above post how does he explain this situation?????
Staff did it for them as I suggested is possible.