Given the language and cultural differences that separate them from everyone else, it's no wonder the wealthy huddle together in urban areas. (Happily, the fact that they do so makes drive-by eggings more convenient.) And it's no surprise that with the rise of a local tycoon class in the late 1800s, a handful of city neighborhoods came to be called "Millionaires' Row," areas where the wealthy concentrated. The most famous of these was the Shadyside stretch of Fifth Avenue, though the term has also been applied to Homewood's Penn Avenue and, as your question notes, North Side's Ridge Avenue.
The North Side's "Millionaires' Row" was unique in that it wasn't part of Pittsburgh when it was established in the early 1890s: The area was originally a separate municipality known as Allegheny. But all the millionaires' rows had one thing in common: They were located just far enough away from the factories that earned the millions for those living inside the homes. As Walter Kidney writes in Allegheny, "Pittsburgh millionaires liked Ridge Avenue, handy to town yet away from its noise and soot (there was more industry in and around the Triangle then), and in an elevated, open setting that was reasonably cool in summer."
Ridge Avenue's heyday was between 1890 and 1910, and not surprisingly, many of the families who moved there during that time were prominent in the iron and steel industries. For example, there was the home of B.F. Jones Jr., who was the son of (take a wild guess) B.F. Jones, who was one-half of the partnership that founded the Jones & Laughlin Steel Company. Not far away was the William Penn Snyder, who owned the Shenango Furnace Company and partnered with Henry W. Oliver, one of Andrew Carnegie's business associates, mining the ore of Lake Superior region. The Byers-Lyon house built at 901 Ridge was built as a double home: One section was built for Alexander Byers, who was -- as the wrought-iron gate surrounding the home suggests -- a manufacturer of iron pipe. The other section housed Byer's daughter and her husband, who no doubt found that having the in-laws right next door enhanced the home's charm. Joining these homes in 1911 was Memorial Hall, intended to be the linchpin of a new campus for the Western Theological Seminary.
But after only 20 years, things started turning downhill for Ridge Avenue and its environs.
In fact, the entire North Side community was beginning to fray after having been annexed by Pittsburgh in 1907. Still sounding bitter decades later, William Rimmel writes in his nostalgic book The Allegheny Story that "No effort was made by the city officials to prevent [the] decay of this beautiful section of the city. They neglected the district. & Allegheny City had been annexed and was now just a poor stepchild." Rimmel also surmises that a merger between two branches of the Presbyterian faith led to the closure of the theological seminary, thus serving as "the final spike in the coffin of Ridge Avenue."
Of course, it's deeply satisfying to blame Presbyterians or city government for our problems. But the blame resides where it usually does: with the indifference of the wealthy. Even Rimmel acknowledges that once the First World War began, "the smoke of the wartime industry added an undesirable factor to the neighborhood," with the result being that these families spent more and more time in their summer homes in the country. As Walter Kidney notes in Pittsburgh's Landmark Architecture, suburbanization was in full swing: Shortly after the Snyder house was built in 1911, "noted residents left Ridge Avenue's urbane but urban environment for country estates" -- located in Sewickley, as often as not. The Snyders themselves fled to the suburbs in 1920, less than a decade after their North Side home was built. As a result, "Many of the houses they left behind were converted to business uses or began to deteriorate."
The neighborhood was both razed and rescued by the Community College of Allegheny County. Construction of CCAC's newer buildings meant the demolition of much of Brighton Avenue. But CCAC also rescued several of the homes that are still standing, turning them to its own use. The B.F. Jones, Byers-Lyon and Chalfant homes were incorporated by the campus, as was the old Western Theological Seminary. The Snyder home and a few others were converted to use by businesses. Now, many of these families' descendents never come to the neighborhood unless it's to get drunk in the luxury boxes at Steelers games.
There, once again, goes the neighborhood.