"It is not possible."
Translation:Det är inte möjligt.
The best thing I can come up with is that while "not possible" and "impossible" are technically synonymous, they stress the severity of a situation differently, and are therefore not exactly the same. They're almost indescribably different because of the subtle implications of each usage. Maybe someone smarter than myself can chime in here, but that's my take on why I don't think they're precisely interchangeable phrases.