Also the reason is

WHM is Earth, Water, Light, Wind based magic.
BLM is Fire, Lightning based magic.
ACN/SCH/SMN spells are non-elemental, as referenced by when you cast Ruin/Broil/Miasma/etc there is a rainbow-aura being channeled into you rather than a colored energy like with Stoneskin or Fire spells. Disease based spells sound like something that would not be associated with an element I guess--thought in past FF games, Bio was associated with Dark element (but sometimes non-elemental or poison-elemental). Come to think of it...are there ANY Dark based spells players can cast?