Theories for this correlation abound. Social scientists have hypothesized that since the PCA was founded in Alabama, which has the foothills of the Appalachians within its northern boundaries, the preference is a nod to local musical history. Historians have pointed out that Presbyterianism and bluegrass share Scottish roots. Calvinist appreciation of old dead people lends itself to a musical genre full of rich history. However, the most prevalent theory is that bluegrass music allows a hip and musical Calvinist a chance to show his/her trendiness without violating the strict taboo of being a "rocker." Nothing is a more sure kiss of death in Christian music circles than to be labeled as "rock." Thus, most Christian musicians, of which Calvinists are a distinct subset, work hard to avoid this label while simultaneously striving to be a hip as possible. This is a very difficult quest but bluegrass provides a nice solution to the problem. Musicians get to play a style that's not commercialized (check), dress differently (check) and assert their superiority over other genres based on very intellectual reasons (check).
For some great, and FREE, music check out: