Soy on soy (on soy, on soy...). Disease soon sets in if not regularly rotated.
But why did they try it
Trying to increase short-term profitability, no doubt. But I don't know for certain.
Why did they try continuous corn? Seed technologies became available that allow it.
My family's been farming corn on corn for 20+ years. Started strip-tilling in the 80's High residue fields help trap as much moisture as we can with limited irrigation capacity. Farmers have no choice but to take care of their soils to remain viable. And more than viable, be profitable as there are many people who depend on them both for their livelihoods as well as an ever demanding population with mouths to feed.