Lack of interest. I mean, even in FFXI where Beastmaster was absurdly good, a lot of people ignored it because of its penalty on party EXP. This basically forced anyone leveling it, for a long time, to have to solo all of their levels.

Not only will it be a limited job, more than likely, but I doubt anyone will really want to play it. Unless it can like... Jug-Pet 7 party slots and solo old fights with relative ease.