Because the issue is specific to the region and started specifically after the tank battle where DU ammunition was used. If it would be a general issue with some dangerous chemical being used, we'd expect to see similiar issues in other regions. Of course it is hypotheticakky possible that at the same time some dangerous and persistant chemical exposure happened in the region, but that is not plausible and also the US would have a strong interest in finding such an alternative explanation. But there isn't any research published, that provided an alternative.

Also look into the wording of the US when sending the ammunition to Ukraine. They state that no radiation hazard is to be expected for the Ukrainians. They do not talk about a toxicological hazard.