Reviewing Chris Walsh’s Cowardice: A Brief History, Kyle Williams offers an answer:
The first and perhaps most visceral is that, short of obscenities, it is one of the nastiest words that can be wielded against someone—and has been for a long time. Cowards are anathema in the Revelation of St. John, among the first to be damned to the lake of fire, and among the most despicable in Dante’s Inferno. Samuel Johnson confirmed the prejudices of ages before and after him when he wrote that cowardice is “always considered as a topic of unlimited and licentious censure, on which all the virulence of reproach may be lawfully exerted.” One wonders what could be worse.
Walsh also suggests that the term provides comfort to Americans. Believing that terrorists are cowardly may assuage the fear of terrorism. “If they were cowardly then they were scared too—vulnerable and weak,” he writes. “And thinking them weak somehow made another new phrase—‘Boston Strong’—seem more convincingly true.” Terrorism targets innocent civilians, relies on secrecy and infiltration. It stands, at least rhetorically, in contrast to the more open apparatus of the American nation-state and its military might.
The Dish 