When I was working as a microbiologist, I did a few experiments growing swabs of HITH bacteria in different agar pHs, and nutrient levels for my own interest as an aquarist, and as a microbiologist knowing the limitations of pH on certain bacterial species.
I found using a high nutrient agar with a pH above 7.5, to 8, the HITH bacteria grew rather well.
Using a lower pH, lower nutrient agar, (pH below 7.5 down to 6.8), HITH bacteria grew , but not as vigorously.
But using an agar with a pH below 6, and low nutrient agar, HITH limited growth severely.
This led me to the conclusion that Oscars kept in the soft, low pH water, with frequent water changes (similar to that of their natural riverine habitat) would be less prone to the disease.
But Oscars kept in harder, high pH water seemed very prone to HITH.
Add the nitrate concentrations accepted by many aquarists as OK, due to less frequent water changes, and you get HITH soup.
Although their may be some validity to genetic predisposition, in some cases due to in-breeding over the last 100 years Oscars have been kept i aquaia,
I lean toward either low quality, or aberrant water parameters compared to what Oscars have evolved in over millennia, to be the main cause.