Because in between the risers the cables are hanging free from the ground, hence the coupling of the cables to the ground is negligible compared to full contact without the risers. Also - outboard crossovers are indeed a thing...So, why would the "cable risers" have any less vibration than the floor? They are solid non-absorbant objects placed on the floor. And, what about all the wiring and electronic components inside the speakers? Seems to me that they are getting more vibrations than anything else.
I'm sorry What Hi-Fi, publishing this nonsense has severely hurt your reputation.