They formed a cartel to not compete with each other to have the lowest emissions, even though that is a feature that some consumers shop for. The idea that the buying public would directly demand low emissions not just the bogieman government, would be dangerous.
As a parallel strategy they agreed to build their DEF systems in a way designed to generate consumer resentment for DEF. They agreed to make the DEF tank small, requiring more frequent refilling, while also making it difficult to refill. Public rejection of DEF technology as inconvenient would lead to pressure on the government to lighten emissions rules.
