It's important to keep things in perspective. The main reason urban cores throughout the "Rust Belt" were abandoned is basically because they were awful places to live. Forget the picturesque European villages and tidy New England hamlets. There is no "golden urban past" when the streets of downtown Cleveland, Pittsburgh, and Detroit were sunny, well-scrubbed, cheerful happy places with neatly manicured gardens and quaint, friendly stores, enjoyed daily by its sophisticated urban masses. There were a few blocks where the wealthiest of wealthy residents lived, and the rest frankly sucked. Go hunt down someone who's 90 or a hundred years old and ask them how nice it was to live in a compact, urban core just blocks from steel mills, smelting plants, and factories dumping pollution into the rivers & lakes and belching black smoke into the sky 24/7.
For that matter, hunt down a 90 year old Manhattan resident, and ask them why people weren't building trendy, upscale lofts in the slaughterhouse/meatpacking district of Manhattan, or Hell's Kitchen, or most of lower Manhattan. Or, head across the Atlantic, and ask an elderly London resident why nobody lived in the Docklands until very, VERY recently.
There's an easy answer why Americans fled cities at the first opportunity: life there utterly sucked for most people. Norman Rockwell's quaint urban America was a fantasy, and people today believe it existed because there's nobody old enough to remember that things were never even close to being that good. Normal Rockwell and his peers depicted an America bearing about as much resemblance to reality as "Beverly Hills 90210" and "Dallas". Or, for that matter, "Miami Vice" (you know, where cops drive sports cars and seem to have more money than the drug dealers they bust, and everyone lives in South Beach and has a tan...)
At its best, city living can be wonderful. But the truth is, 99% of the people who intentionally move to urban cores NOW from suburbia have incomes and lifestyles that only the wealthiest of wealthy city dwellers could have even fantasized about a century ago. For just about everyone else -- forced to live in the city by necessity -- the quality of life wasn't a whole lot better than it is in most working-class inner-ring suburbs today. And make no mistake... part of the reason why Americans who leave suburbia to move to urban cores NOW can actually afford to live the way they do is because everyone else moved to suburbia and made room for them there.
As for government policies, I'd say Rural Electrification and highway construction are the two obvious ones. Civil Defense strategies of encouraging people to disperse across large areas to minimize fatalities during a nuclear war come to mind, too. Not to mention VA and FHA loans that made it possible for non-wealthy people to afford to buy their own homes, partly justified by the belief that widespread home ownership was an effective way of nipping communist and socialist leanings in the bud. It's easy for voters in a city where nearly everyone rents to vote to increase property taxes to fund social programs, especially if most of the renters live in government-provided housing and don't have to bear the costs of increased taxation anyway... it's next to impossible to raise property taxes in cities where nearly everyone is a homeowner and will directly feel the impact of every last cent.