because our drive shaft is too stiff torsionally.
It is sure that the current flexible element, a Drivesaver, is pretty stiff, torsionally and radially, than the Centaflex.

I have been reading about such issues a lot in the past few days, and the only conclusion is that it is a black art, unless you have a load of experts, a few months and a big computer to analyse the issue mathematically.
I do know that car manufacturers work on this and experiment with new models, sometimes several pre-production modifications before the drive train runs quietly. Of course, when engineering for a million model production run, there is lots f money for trial and error