Well, it depends on which ftp daemon that you're using, but I know that, for example, in ProFTPD you can specify in the proftpd.conf file what you want the lowest
uid and gid to be that can log in. And if these directives have been set, it's certain to be set higher than 0.
Would you really want root to be able to log in for FTP, or are you just curious as to why it didn't work?
Have to admit though, I've never tried ftping to myself, could well be that the daemon would not allow it.