Probably one of the reasons people want to describe them as cowardly is because it is a given that among their own people they will be portrayed as heroes full of bravery, which is the opposite of being a coward.

It is also that we have become so lazy with our adjectives. My dad, granddad, or even The Virginian (:-) would have been much more colourful with something more along the lines of yellow-bellied, lily-livered, side-winding, no-account, worthless son of a gun. Even that is not as eloquent as they would be as I do not have their skill. Theirs was a world of language - the most important medium. Now, we live in a world of images and sound bytes and it is our loss, imo.

In my grandfather's old dictionary set from 1894, The Modern World Dictionary of the English Language, it gives the general term of being timid, cowering, tail-between-the-legs definition, but it also lists to make coward, to intimidate. To illustrate it, they use a quote from Henry V: "That hath so cowarded and chased your blood
Out of appearance?"

Obscure, I know, but still interesting.

Other synonyms listed include: pusillanimous, craven, faint-hearted, timid, spiritless, mean...