Ducati got rid of steel GP frames, I figure, b/c they couldn't be consistent enough. Stoner said each steel frame felt very different from the next. Not sure its a material issue, may be the welding and cutting precision. With carbon, the design was not so easily changeable in terms of its lateral rigidity.
This ^^^ The reason they moved away from the trellis was specific to not being able to reproduce the same feel. many assumed that it was because of the welds.