While Pittsburgh is not racially diverse, the place was as ethnically diverse as they come. In fact, many attribute the existence of so many ethnic enclaves for the reason widespread decay was fairly slow to happen.
Here is my take on the differences between what happened in Cleveland, vs what happened in Pittsburgh.
1. Downtown. Pittsburgh's Downtown is hemmed in by the rivers, and surrounding hills. In many ways, the forces that shaped Pittsburgh's skyline, are the same as in Manhattan, but of course on a smaller level. A lack of cheap, buildable land, forced builders to build upwards, while Cleveland had plenty of room to spread out.
2. Neighborhoods. Cleveland is overwhelmingly a city of frame houses. Frame houses tend not to withstand neglect, thus an area of frame houses will quickly go to seed if care and investment are lacking. Pittsburgh's most vulnerable areas tended to be filled with brick buildings, including many rowhouses. Not needing to be painted, these can withstand much more neglect. Those of you in Cincy need only look at how intact OTR is, despite how long it's been "the hood".
3. Industry. An earlier poster was right. Pittsburgh lacked diversity in it's manufacturing. Steel was overwhelmingly dominant, and when it crashed, Pittsburgh ceased to be an industrial city. Cleveland has held on to much more manufacturing, but has been enduring a slow bleed.
4. Higher Education. While Case Western can be compared to Carnegie-Mellon, Cleveland State is in no way comparable to Pitt, with it's huge medical complex, and AAU status. Pittsburgh also has a major, 10,000 student Catholic university (Duquesne), something Cleveland lacks. Along with Point Park University, and 2 women's universities (Carlow and Chatham), these employ a ton of workers in Pittsburgh, and are in many ways, the backbone of the economy here, and that's not even counting Robert Morris University, which has a campus downtown, but is mainly out by the airport.