By 1918 Maori had been interacting, intermarrying, interbreeding with settled Europeans for more than 120 years. There had been a lot of dying thanks to vulnerability to introduced diseases in line with other new world populations in the preceding 150 years. But this wasn't due to a genetic propensity, it was due to a lack of exposure. By 1918 this period of high disease susceptibility had long past - by 70-100 years.
But speculative genetic propensities aside, Maori had all the requirements for a good influenza outbreak: They tended to be poorer, less exposed to adequate health care, less educated, and they lived in closer knit communal groups that included extended families and relations. I'm not sure it is necessary to look beyond this for reasons why a community might suffer more. But *shrug*