Why? You have to ask those sites, not us. Imo, it's not for Noobs. I've seen many threads here that someone managed to format his WBFS-Partition and lost all his games (and was too stupid to use a Command-Line Tool to restore them), or don't know how to partition the drive, for example.
FAT32/NTFS may be easier for the Noobs to handle, and are supported by nearly all OS (FAT32 even with all Homebrew-Apps), that's the only reason for me.