A steward (or manager) is someone who handles and oversees someone else's possessions. They're responsible for them and their use, but they don't own them, don't value them intrinsically the way their employers do. It makes sense, then, that a "steward" in the Bible represents the "external Church" -- the part of a church that oversees and enforces ritual and outward behavior, but has no thought for the good wishes and understanding that are meant to lie within those external forms of worship.