Like I've said in other threads, this fight was never going to happen. All it's taken is for GSP to win a majorly hyped fight against Shields by just pawing his way to victory against a bad striker, and his trainers have finally thought 'sod it, we're not comfortable putting him in with Silva, he's starting to not look good by not finishing WW fights, let's knock this one on the head.'
The weight thing isn't a mere excuse anyway, they are not similar enough in size/natural weight. If they fought, one of them would not be in optimal condition, which is the last thing you want in a 'superfight'.
Silva dances around when weighing in at over 180. Imagine he had to drop more weight? He'd be floating on air
meanwhile GSP will be walking around feeling like a sumo wrestler, totally ruining his explosive takedowns. It would be a total mismatch, and, even light as a feather, Silva would pick GSP apart.