An intelligence based tank would severely upset gear distribution and balancing, I'd rather we keep the main stats for tanks and healers as is.

Also, aside from NIN that eventually branched out of MNK gear, all jobs that were released with expansions use already available gear sets. DRK shares gear with other tanks, AST with other healers, MCH with BRD, RDM with caster DPS and SAM with MNK.