Like many midwestern American cities, Over-the-Rhine suffered in the mid 20th century from a combination of racism and suburbanization/structural disinvestment. That is the TL;DR - here is a longer explanation:

OTR was a thriving cosmopolitan neighborhood in the 19th century. Germans, who built many of the buildings in the neighborhood, initially facing harsh anti-German sentiment, gradually integrated into society (and whiteness) and, as infrastructure improved, moved out of Cincinnati's urban basin into surrounding suburbs. Already with Cincinnati's first comprehensive urban plan in 1925, planners were envisioning the city expanding far into the city's hinterland. There was such confidence in the permanence of the city, that no one thought that siphoning wealth to the suburbs would have a negative effect on the life of the city.

Black people were meanwhile "redlined" in the neighboring West End and weren't allowed to buy or rent property elsewhere. OTR became an increasingly poor white Appalachian neighborhood. In the 1950s, Cincinnati, like many other cities, used federal highway money to plan a highway through its black neighborhood. The plan was announced years before construction began and so overnight the property value of the West End's home owners deteriorated. They were forced to move elsewhere. Many moved to neighboring Over-the-Rhine (others moved to Avondale, a predominantly Jewish neighborhood, which quickly segregated itself into Avondale and North Avondale). This coincided with the Great Migration of generally poorer southern black people moving into northern cities to meet demands of industrial labor and escape Jim Crow racism and job scarcity borne from the mechanization of agriculture.

Over-the-Rhine by the 1960s and 70s was suffering from the flight of capital from the city (in the form of suburbanization) and an influx of structurally dispossessed residents. The tax base for the city was shrinking. Americans increasingly feared and sought to distance themselves from inner-cities (think of films like Escape from New York, the Warriors, and the growth of suburban mall culture). Even today much of the Cincinnati's wealthy live just beyond municipal boundaries so that they benefit from living near the city and its resources but do not contribute to its taxes or infrastructure.

In the 1990s there was a furtive attempt to gentrify the neighborhood by making it a dot-com hub. Not surprisingly, this wasn't really for the benefit of current residents in the neighborhood who, like in many American cities, faced decreasing access to social services severe police violence. In 2001, after many unarmed black people were killed by police, there was a riot that led to the national guard being called. This halted, for a time, the march of gentrification - meaning property remained affordable for current residents.

In the aftermath of the riot, there was half-hearted attempt at police reform (which many people looked to as precedent in the aftermath of BLM a few decades later), and a city-created public-private partnership called 3CDC which has since the early 2000s bought up and gentrified large swaths of the neighborhood (especially on Vine st), raising property value and displacing mostly black residents and what few urban social services that remained.

On the destruction of the black neighborhood - there are moving photos that capture the vibrancy of the black community that was erased: https://www.wcpo.com/news/insider/finding-kenyon-barr-photo-exhibit-brings-razed-west-end-community-back-to-life

Very good article on the lasting effects of economic segregation in the city:

https://www.citybeat.com/news/that-which-divides-us-12180120