Why? The land is there, the auto is there, the race and immigrant problems of the cities are still there. Anyhow, suburbanization began in the US in the 20s. Also, suburbs weren't mostly just people fleeing the city. In the big picture, demographically it was rural people transitioning to a form of urbanization that they found more congenial.
That said, here are a few thoughts that could make some difference.
1) Butterfly the Warren Court. A lot of its rulings re criminals and the precise scope of desegregation are contingent. Without them, you will probably have more vibrant de facto 'gated' urban communities.
2) Make condominiums an accepted and recognized form of ownership much earlier. There is no particular reason they couldn't be, which means that the legal and societal push for ownership for all won't as much push in favor of the suburbs.
3) some kind of petroleum bug that attacks gas supplies, making travel more difficult and expensive? Otherwise autos are pretty much inevitable technologically and industrially and because voters vote, highway infrastructure is also inevitable.
4) some kind of effective area defense against nuclear attack, coupled with a some actual nuclear exchanges to drive home the point