The difference with dieselgate is that the cheating was known by European regulators, who did not bother enforcing the law.

https://www.theguardian.com/environment/2016/jun/20/european...

VW was surprised by the American response, since they simply did not expect the law to be enforced (they were apparently unaware of a similar diesel scandal that occurred with US truck manufacturers in the 90’s).

Source: Faster, Higher, Farther: The Inside Story of the Volkswagen Scandal Book by Jack Ewing