One of the major reasons for this is because the community's so small now. Making a base of 4 and 8 people means that they can balance content that's relevant for the players right now and then expand upon it in the future if (when!) the game becomes more popular as it's improved.

I'd expect to see alliance content make a come-back eventually once 4 and 8-man is fleshed out. Personally I quite like the idea of 4-manning things, it'll make it easier to just get friends together to do things and feel like a real adventuring squad like FF's of old!