Why are Y capacitors better for common mode noise?

 

primary switching noise is injected via the inter winding capacitance of isolating transformers, creating common mode noise in the secondaries. these noise currents are difficult to filter & travel long distances, enclosing large loop areas thereby creating emissions problem.
So Y capacitors are used to provide a local return path for these currents with small capacitors connected between the secondary ground and one of the primary power rails.these capacitors also help any filters on the secondaries to work much better by reducing the source impedance of the emissions so that common mode chokes can function effectively.

 

So why only Y capacitors and not X? What is so special about the materials in Y to allow for such a return path versus others? Is it just because Y have a very high storage capability?

Is it therefore only ever the case to try to use Y capacitors near switching sources? I have a design where the designer tied two secondary grounds together with a Y capacitor.