Interesting question to discuss but first I want to ask was you from El Paso? (I forgot which city) I'm going to speak on the black gangs first then city crimes later.

Black gangs are territorial due to the drug trade and community identity. Drug markets vary from city to city yet the operational methods are about the same. It's operated like a corner store or fast food franchise which relies on location and customers. Therefore, if a gang is making tens of thousands of dollars weekly or more than their business is lucrative. Just like any legit business it's about location. Now if competition set up shop near the area than customers will be pulled away, which brings either killings or mergers into place. Community identity or pride goes along with it because that's where they originated or lived and that's with everybody. NYers have their boroughs beef (Brooklyn vs Queens), Chitown different sides/communities ( West side vs South side), Next door cities high school sports teams, etc.

Fragmentation varies tremendously from group to group. The causes are different leaderships, personal affairs,greed, informants, and law arrests/indicents. Monolithic groups/organizations have been the incorrect view of many groups such as Crips, Bloods, Surenos, Nortenos, 18st, Folks/People Nation, UBN, and even La Cosa Nostra. It's just the umbrella identity and every group is independent. GDs have suffered fragmentation the heaviest in Chicago due to indictments & internal leadership. Crips have been the football kicked in many conversations as examples of fragmented groups but that's completely false since some gangs existed before the formation and after it was meant to be a partnership among different gangs to resolve conflicts not a monolithic syndicate.

Most of the time it's true that the top ten cities have a major black percentage ( my state capitol Jackson is 70-79% black) but all those cities share a similar historic economic decline and drug trade boom. Same situation repeated nearly nationwide. But have notice that almost the same cities had the infamous reputation BEFORE they was majority black ? Not trying to start a ruckus just saying study the history of different cities crime past and look at demography then and you'll see that the cities reputation have already been established. Besides there are too many black majority communities to name that have average to low crime, they're just not newsworthy