Because stuff like rebuilding after natural disasters increases GNI, as well as having lots of people in hospital. Yeah the GNI is raising, but it's not exactly a sign of development.
But the biggest problem is not accounting for the income gap within a country. Like, there are cities in China that are far more economically prosperous than most places in the world, while having rural areas stricken with poverty.
"I'll make sure to sin in as many entertaining ways as possible before I die." - Sheogorath