As has been already mentioned, RDM is one of those Jobs that just wouldn't fit into ARR's narrow role-focused battle system. After all, in FFXI it had be constantly tweaked - first with the enhancing elemental spells on it's weapon, and then being turned into a weird hybrid mana battery with Convert and Refresh. The 'jack-of-all-trades, master-of-none' schtick just wouldn't fit into ARR sadly, where Jobs and their base classes require dedicated roles. Not to mention a lot of RDM abilities are already in use by other classes (Convert on BLM for instance), making RDM very much an orphan.