His eyes would have to be in the same place as they were on the previous model and everything else that changed would have to work around that.
Because everything was done so precisely, the production would have been limited in the way that the new Sonic character could be designed.
The fact that the Sonic the Hedgehog movie worked so meticulously to make sure that James Marsdan was always looking in the right place actually makes the visual redesign of the Sonic character that much more interesting.