The correct answer is because the nickel is not big enough.

So we'd need way more ferrous material than that for the ice wall to be ferromagnetic.

The Engineer and I came up with a nifty little theory to explain this.

According to modern physics, apparently, the universe can be described in two ways... with everything larger than the planck constant, and measured with 'big' strings wrapped around calabizoodle spaces, or with everything smaller than the planck constand and measured with 'small' strings stuck inside clabizoodle spaces.

Obviously, then, the nickel is the 'small' kind. So there could be tons and tons of it in a single atom of ice.