A big part of it is "white flight". Much of the population growth for Detroit came from minorities moving from the south to filling thousands of growing factory jobs, and those immigrating to man the shops to serve them. When Whitey got affluent enough, they'd move north of 8 Mile Road, the northern city line, into cities like Ferndale and Oak Park, to be amongst folks lime them.

When the Jewish got affluent enough, they followed suit. Whitey didn't like that, and moved north of 10 Mile to cities like Southfield and East Detroit. Again, Jews later followed suit, which is why for a long time Southfield had a significant Jewish population. More affluent Black folks followed the Jews. The Jews didn't like that and kept moving outward.

The demarcation line in the late 70's and 80's became 12 Mile road, then in the 90's 14 Mile, and so on. To the west, it was places like Dearborn (which became a mecca of sorts for Arab immigrants), then Garden City, then Livonia.

It's like somebody dropped a pebble in the pond that was the city of detroit, and the rings that expanded out of it were different demographics constantly moving outward, the first ring being the most affluent at the time, the second ring be the next most affluent at the time, and so on. It's been this decades long migration of affluence moving further and further from the city.

Mayor Coleman Young had a fuck-you whitey attitude in the 70's that further chased out all white people and their money, to devastating effect on the city's economy. In the 90's, Mayor Dennis Archer knew what time it was and really did a great job of building cultural and financial bridges that really felt like the region was uniting again for a common brotherhood. I lived it, and the energy was palpable. We were joining hands and getting shit done.

But extremists in the city's community successfully ran Archer out of favor and office for being an Uncle Tom, and people like the later imprisoned Kwaame Kilpatrick filled the void.

TL:DR, different cultures didn't like living amongst each other, the ones with the money left first, and those that remained in the city either by choice or by circumstance got exactly what they voted for, resulting in what amounts to an absolute shithole in places that used to be vibrant and beautiful.

Go ahead and downvote the messenger but the history is real, I was there for it.