One reason is because SE has simply failed at job balance for high-level content. Some jobs are used a lot, some are used occasionally, some are one-ability wonders, and some just have no place anywhere. Why bring Job X when Job Y is just flat out better in every way? Unlike other games, content doesn't allow for all forms of damage, tanking, and healing to come out effectively even so lots and lots of jobs are just left out in the cold and SE can't/won't do much about it.
Bastok Rank 10
MNK 99 / BST 99 / SMN 99 / DNC 99 / SAM 99 / WAR 99 / THF 99