[niky]

Because they weren't tested in the US to US standards.

While the European, Australian, Chinese (their crash testing speed has now officially been raised to the same level as Euro-NCAP) and now, the ASEAN market all accept the same NCAP standards for crash-testing, the US still has its own specific, bespoke, standards, that are different from everyone else's.

As seen with issues with BMW in the past... building a car to one set of crash standards does not ensure it will meet the same results when tested under another. This could be due to specific reinforcement needed for certain speeds, or airbag activation schemes, or any number of things.