Kala Sarpa is dated here as modern, and the more interesting question is why a combination absent from the classical literature became one of the most talked about in contemporary practice.
Three things account for most of it. The first is that it is easy to see. All seven classical planets on one side of the Rahu Ketu axis is a single visual check on a chart wheel, requiring no house lordships, no strength assessment and no period calculation. In a market where readings are given quickly and often at a distance, a rule that can be applied at a glance has an advantage.
The second is that it is dramatic, and its name is dramatic: the serpent of time, an entire chart swallowed. Combinations promising a difficult life attract attention in a way that a modest Dhana Yoga does not.
The third is commercial. Remedial services offered specifically for this configuration are widely advertised, and a diagnosis with a remedy attached to it propagates faster than one without.
None of that makes the configuration meaningless within its own system; a modern rule is not thereby a bad one. It does mean a reader should know it is modern, that classical sources do not contain it, and that its currency owes something to how it circulates. The atlas records it with that dating attached.