First of all I want to make clear that I always liked FTP. And I still prefer it over uploading my files through a web-interface, but I now have found quite a big reason why FTP really really sucks.

Many users prefer using a nice GUI for FTP-transfers instead of good, old command-line FTP.
Since this usually involves installing an extra program which could become unnecessary with this cool Firefox extension: FireFTP. Many users use Firefox already, so why not also use it for FTP?