They had to normalize the damage of the elemental spells to keep things fair.
This^

Yoshi-P also commented on the fact that because different classes have different spell toolkits, it wouldn't be fair for one class to be able to kill a mob with one spell cast and one class not be to kill without a fight for their life, if they're even able to harm it at all.

The elemental resistances were removed for game balance, which I can agree with. I hated encounters/ battles in FFXI where BLM was the only way to go because they were so superior to other DPS in every way.