Because other countries have—in general—a demonstrably poorer track record than major airlines in the U.S. for a wide variety of factors, including poorer training programs, maintenance regimes, operational expectations, and sometimes cultural norms that interfere with good CRM (crew resource management).

I'll address the elephant in the room and say that I don't think it's coincidence that the two airframe losses in the MAX 9 were with Indonesian and Ethiopian carriers and not American or European ones, despite American and European carriers having received—as far as I can tell—the bulk of the deliveries of this airframe and at least a plurality (if not majority) of hours in use (if someone else can find good data on this I'd be grateful). I suspect that more resources put toward pilot training played a meaningful difference here. For example, the first officer of the Ethiopian flight had less than 200 hours of flight experience. This would be completely unacceptable for a U.S. major.

I want to be clear that I am not saying that it's the pilots' fault, but that the swiss-cheese model relies on multiple overlapping failures. The additional resources spent on training, maintenance, operations, and CRM at American and European airlines in general ensures fewer overlaps and thus fewer incidents.

After watching Downfall, Lion air asked multiple times for additional training but was refused as this would cost boeing money. To me, that looks like Lion Air was more aware of issues than US operators. Boeing never even bothered to inform pilots of the MCAS system's existence before the first grounding.