[sub]I'd imagine that it'd have something to do with the "structure"

Liquid water is... well, a liquid, and flows freely, meaning the method of controlling it is significantly different from Ice, which is rigid.

Ice requires less mass to take up the same amount of space, meaning that the focus of the magic is different. Most of water magic's mana usage comes from creating the water, while most of ice magic's focus is on shaping it.

This also plays into their strengths and weaknesses. Water magic has more material to work with, making it better suited towards healing for longer periods of time or larger areas. Ice magic can produce pointed shapes and has a low creation cost, making it better suited towards combat.

'Course, this could also NOT be why, but hell if I know.[/sub]