IIRC, its just the way the antennas connections are set up. the baseband they use supports it, but their antenna setup isn't physically capable of SVDO.
The reason why voice/data works on the EvoLTE and the Sprint Variant of the Galaxy SIII is because they have SVDO chips which allowed that to happen.
With the Note II, it appears as Samsung wanted to have little to no hardware variation amongst the different handsets, hence the no SVDO chip.
My Evo LTE was capable of simultaneous voice and data, even on 3g. Also, I'm almost certain the Galaxy S3 is also capable.