A couple reasons. One, I believe the reason soybean was chosen for this study was simply its prevalence in US food -- not because it's special or unique. As an oil it shares characteristics with a number of others, and the most notable of these characteristics is its fat profile (>50% PUFA).

Two, a connection between high PUFA oils in general and poor health outcomes is being postulated by other researchers... and, to be fair, many people who probably have no particular expertise and/or hidden associations, such as diet fads. But that's one of the reasons I asked the question initially; even the sources that would normally be trustworthy on other science (government scientists) have proven to be wildly incorrect and untrustworthy when it comes to dietary issues. I was honestly hoping someone that works in the field would pop up to comment.