I guess this is partially to do with the grain of reed instruments - how they're more closely related to the human voice, and consequently more capable of intimate expression. Maybe. Brass instruments have a purer, more strident, more 'elemental' sound (never mind that they're about three times louder).
Bearing that in mind, where the strings at? Are they just too demure for the purposes of hook-hungry samplers? Then again, bowed strings have never featured prominently in African American music; they're too quiet, and the attack of a bow is not conducive to the sort of percussive, heavily syncopated playing that predominates. Then again again, they also have certain European, bourgeouis connotations; chicken/egg?