Sheldon Imaoka
Ansys Employee

It is related to implementation - VCCT method is based on energy release rate, which supports anisotropic elastic materials whereas SMART method is based on J-integral or stress intensity factors, which are limited to isotropic materials.
(Although VCCT method supports orthotropic/anisotropic materials, you do need to be careful since, if your crack is not aligned with the material direction, you could get non-physical results. It is typically assumed that the crack is aligned with material orientation, so that is why anisotropic/orthotropic may be used, but it's not completely general in nature.)
Regards Sheldon