No radar att Kiruna tower
When the plane was handed over from Sweden ctrl, the radar service was terminated.
According to the approach plate for runway 21 at Kiruna the lowest safe altitude there is 5300 ft, which is probably what they were cleared to descend to.
Focusing on the approach plate and being used to radar vectors, it is likely they forgot that ATC may clear you to an altitude that is below the MSA at your present position. Even if they were cleared to the airport MSA of 5300 ft, it was their own responsibility to make sure they were within 25 nm before doing that.
The crash happened about 40 nm out, where the MSA is 9000 ft.