Potentially, its because of the Job quest itself. In one fell swoop, if they started it at 50, then they have to have a story for Heavensward, Stormblood, and Shadowbringers. Red and Sam ended up only with their initial two story arcs (Heavensward and Stormblood). AST, MCH, DRK (ARR and Heavensward). So I kinda suspect they simply don't want to make three tiers of story/work on top of all the rest of the stuff they need to do with the established class/jobs as is.