This was a deliberate decision by Yoshi-P, mainly because he was afraid people would pressure others into making it mandatory.

Well, voice chat was mandatory in my WoW guild for all raids, and this was before it was implemented into the game. We were required to use Ventrilo or we didn't get an invite.

See the full translated post here:

http://forum.square-enix.com/ffxiv/t...l=1#post544090