Carbs became 'good' because fat became 'bad'.
In the 50s, the US were very concerned about mortality rates from heart disease and they were trying to find a cause. Obesity levels were pretty low at that point in time.
(this is a very condensed history by the way!)
Scientist called Ancel Keys conducted 'the Seven Countries' study, claiming a link between fat and mortality rates, and that sealed the deal. It became a political and media issue, rather more than a health one, and from then on, fat was something that we were all told to avoid.
The actual data that he had, however, was from 22 or 23 countries (can't remember exactly) but the correlation could only be made in the 7, so he conveniently ignored the rest.
Gary Taubes book 'The Diet Delusion' is a brilliant account of how we moved away from fat and started to base our diets on carbs - and the obesity that has resulted ever since.
If you have a look on the spreadsheet, there's a brilliant video from The Diet Doctor where he shows slides of the US showing how obesity rates increased, state by state, from the early 80s when the 'fat is bad' message became totally mainstream.