At a guess, because Wimmer's suited to a back four - but certainly not for a back four. Of course the question that's worth asking about that is why not have Wimmer in the centre with Verts and Toby either side of him. it even makes sense with Dier dropping back to cover.
Looks like you think he's not suited to any number of defenders! I know what you mean, of course. He's played left-back for Switzerland and I think he could play on the left of a three. As you point out, he could also play the central role. Probably better, as he looks well out of his comfort zone as a fullback.
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