I think that members of the Weather Underfound might also have been discouraged by the dissolution of the Red Army Faction in Germany, dubbed by journalists the Baader-Meinhof gang. (Though the main leaders of the group were Andreas Baader and Gudrun Ensslin, Ulrike Meinhof received a lot of publicity, partly because she wrote the group manifesto.)
Most leaders of the group were captured in 1972. Ulrike Meinhof was found hung in her prison cell in 1976, and Andreas Baader and two others were found dead in their cells in 1977. Most observers believe the official government verdict of suicide.
In a similar way, the Red Brigades in Italy were the most active in the 1970s (1981 marked their last well-publicized terrorist action, if I remember correctly.) The organization has been considered inactive since about 1988.
Japan had a similar group whose name eludes me (Red Star?) I think that they also were most active in the 1970s.
Workers of the world, unite! You have nothing to lose but your chains.