Because they knew it upfront. The whole Diesel testing system was rigged to eliminate costly end machine testing as done in the US, which historically led to costly cheating on the roll by the US manifacturers. They paid experts passing those tests which were paid as much as a Formula 1 driver.
So the Europeans improved that to reproducible test cases, and much cheaper, only with the engine.
So when the problem with the overly large AdBlue tanks came up, the Europeans knew they can cheat that away, because legally they thought they can get away with it. The engine test passed legally, and there was no legal framework to protect actual sold cars from emitting too much NOx.
This is of course entirely the fault of the politicians, which were of course lobbied into that, and didn't ask much.
That's why VW insisted all the time that they acted legally in Europe. In the US of course not, because in the US the big three always had the advantage of special drivers to pass the emission tests.
So they still don't fear any legal woes in Europe.
So the strategy is to keep calm, and protect the rest of the industry, the politicians, the workers, change the ECU's, and either add bigger tanks or change AdBlue more often. It's insane to ask for 2 years AdBlue storage anyway. 6 months if enough.
And the good engines can pass the new EURO 6 limits, because their injection system is highly optimized. Those engines are miles better and more effective than a normal otto, which enables them to actually use the crap diesel oil. Problematic are only the overly large engines or the cheap ones.