Because there is no way to establish relations with people across servers, using Duty Finder exclusively doesn't facilitate meeting people in ways that create positive long term relationships in game. That in turn builds a social barrier preventing a lot of people from tackling 2nd Coil of Bahamut because raiding with strangers is stress inducing instead of fun.

Perhaps giving people allied seals for completing dungeons with a set group from their own server would help push things more in the right direction.