Balance in HotA.

The icons for Flight, Dimension Door, and Teleportation are clearly designed in the color scheme of their respective elements, so if they are moved without changing the icons, these spells will look strange and out of place within the "frames" of other elements (and will simply appear as air and earth spells unless you specifically remember that the icons have been changed).
The gameplay will have the side effect that each of the 4 elemental books will effectively provide full control on its own. This looks a bit messy, and it's unlikely that this is what you want to achieve. In the case of adding new control spells, each book provides something unique, rather than everything at once.
Logically, only Flight is really associated with air, while Dimension Door and Teleportation are not particularly associated with anything (especially since Teleportation is used in water in some Might and Magic games).