Why faith, as a grace, in particular

Look at this simple passage from Thomas Watson's Body of Divinity. It's one of those striking things that when you see it explained like this you wonder why you never thought of it before:

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What is the condition of the covenant of grace?

The main condition is faith.

Why is faith more the condition of the new covenant than any other grace?

To exclude all glorying in the creature. Faith is a humble grace. If repentance or works were the condition of the covenant, a man would say, It is my righteousness that has saved me; but if it be of faith, where is boasting? Faith fetches all from Christ, and gives all the glory to Christ; it is a most humble grace. Hence it is that God has singled out this grace to be the condition of the covenant. [emphasis mine]
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What I'm getting at is not the fact that with faith is no boasting (vs. works, etc.) and all that. That is basic doctrine. What I'm getting at is even *thinking* about why 'faith' is *the particular grace* (and not any other) that is the condition of being in the covenant of grace (basically of being saved).

In other words, that word faith we just kind of say; and it gets knocked around, of course, by the atheists and others (blind faith, no evidence, etc.), and as a believer one just basically accepts the word and the act. But it seems vaguely abstract still. Yet we can see it more practically with the above explanation from Watson.

(And, again, for any who don't know me, I am not having a bell rung in my head over the fact that faith is different from repentance or works. The point is, basic doctrine aside, it's unusual to think 'why faith rather than any other grace.' Christians don't generally have that question enter our minds. Not in the way Watson presents it above.)