Why is D12 the poorest district?

On the contrary, as someone born and raised in Utah, it makes so much sense, and I can’t imagine a more apt city choice than salt lake. The Mormon church is headquartered in SLC. They have an investment portfolio of over $100 billion. They own large swathes of land throughout the United States. The FBI and CIA both heavily recruit Mormons. In the hypothetical HG future they would lose the religious trappings in favor of other subjugation methods, but the Mormon church absolutely has the core amount of power and influence that the Capitol would need to dominate the rest of the nation after the fall of the USA and apocalyptic events described in the series.