Over the course of the 20th century automobile companies did all sorts of stuff, some legal, some illegal, to sabotage public transportation. Selling individual cars to individual people (or at least to families) is highly profitable. I mean, buses and trains carry tons of people for free! All those people could be paying money to get their own private cars. That's profit to be made!

Source: http://www.brooklynrail.net/NationalCityLinesConspiracy.html

1972 NYT article of Sen. Ted Kennedy calling for an investigation of GM on grounds that it "guided the development of bus and commuter rail transportation consistent with its fundamental interest in selling private automobiles."