Because the prism would have to be put on first. That means you would have to know what blade is coming and prepare for it. And then what will happen to stacked blades? I am a Pyromancer so I will use a Fire example. If I put an Ice to Fire Prism on myself and then cast Elemental Blades and then cast a Fire blade, when I cast my first fire attack it would be converted to Ice and would not activate the fire blades at all. I would end up having to cast a second Fire spell in order for the fire blades to activate. AND if that prism was put on AFTER the Elementals and Fire blade then the Ice blade in the Elemental Blades would never activate at all. This could only ever be helpful if it was a spell that allowed an opponent to be targeted. That way we could convert them to something that we were more resistant to. Or force them to avoid all the traps they just put on us.
