complexity theory – Why is it hard to show that the euclidean Steiner tree problem is in NP?


I read that for the euclidean Steiner tree problem it is known that it is NP-hard, but not known whether it is in NP or not. [Wikipedia]

Shouldn’t the euclidean version obviously be in NP since the metric Steiner tree problem is in NP and a non-deterministic TM that decides the metric Steiner tree problem could also decide the Euclidean Steiner tree problem? Or am I missing something that could make the Euclidean version inherently more difficult/different?