The article doesn't really go into why the reservations were worse hit than other US areas. They are "sovereign" "nations" so surely they have some autonomy to set their own lockdown, travel policies, and other responses to the pandemic. Did they have even worse governance than the US generally? Or dense living conditions? Or particularly vulnerable age profiles? I get that they're not generally wealthy, but probably…
Reservations basically operate with only the local government. Imagine your town trying to source all the required goods themselves while there's a supply shortage.