I'm currently reading Days of Rage, which is about the Weather Underground. Two points stick out to me:

1) First, the racial component. According to interviews of WU leaders, their main object was violent revolution against white supremacy. The author traces their ideological history through black militants, finally ending in an uneasy alliance with the Black Panthers. In looking back across American history, virtually all (excepting, probably, the anarchist wave of violence) militant organizations (right and left) had race at or very near the center of their program. The KKK was arguably the most successful, having wrested back political control in the South after a campaign of violence and terrorism.

2) The WU were amateurs who had an intellectual affinity for violence and terrorism, but had a bourgeois rejection against it in practice and were incredibly unsophisticated about their operations. It seemed almost as a privileged interest in terrorism rather than revolutionary commitment. In a society as large and materially wealthy as the U.S. it's probably difficult to make any serious pitches to commit oneself to revolutionary violence.