I blame it on the greed and short-sightedness of both the management and the labor unions.
The management thought that it could simply make whatever kind of car it wanted to and that the public would just buy it regardless of quality or fuel efficiency.
The unions wanted extra generous payments, and conditions which were unsustainable, such as retirement after 30 years, which meant that one could start work at 18, retire at 48, and collect a pension plus full health benefits for the rest of their lives.
There is a LOT more to say about this, but I will hold off on any "rant" until others have had their say as well.