Why can't I create a directory under /opt without sudo?
Only the root user can write to the root of the file system. So, it is safe to assume that the /opt directory is owned by root. The default umask would make that directory read, write, and executable to the owner;
read and executable to the group; read and executable to others/everyone.

If you use sudo mkdir /opt/bupo the directory would be owned by the root user.