Not sure, but I think it's because the SpectraVision allows for hardware calibration, and the included software is designed for their own calibration tool (eye one). Problem with the Eye One Display 2, is that they have a tendency to die rather quickly. I had two in 5 years, both had the same calibration issue after 2.5 years, and I am not the only one who had the problem.
Since then I switched to a DTP94 calibration device.
Anyway you cannot go wrong with NEC, but I could recommend Eizo and Quato as well