In humans, enlargement of internal organs (including heart) is know to be a side effect of using growth hormone as a PED. Wiki says [0] that "Extracted growth hormone was used since the late 1950s", so in 1973 it was already a well known technology.
[0] https://en.wikipedia.org/wiki/Growth_hormone_therapy#Extract...
No, Occam's Razor suggests the simplest possible explanation. To suggest that they figured out how to dope horses in 1973 and then promptly forgot how for the next fifty years is a complicated explanation. The simplest explanation, the Occam's Razor explanation, would just be that it was a fluke of nature.
Maybe anti-doping rules enforcement became stricter?
Horse doping is old. Schwarzenegger was doped up and winning in 1968+ so the science was there.
The idea that someone wouldn't dope up a horse for a marquis event like this is absurd. And once Secretariat became that famous, nobody across the industry was going to kill the golden goose by pointing out doping.
However, the window was narrow. New tests came online in the mid 1980s to detect horse doping.
The fact that nobody has been able to get even close to Secretariat's times given significant advances in nutrition and training is extremely suspicious.