...simplicity. It's wrapped up in the glass tips and not in the air stream anyways so it doesn't really matter what shape it is as long as it does it job and fits in the elevator ends.
As an amateur aircraft developer, here's my take on it:
Every airplane ever built consists of a conglomeration of compromises all flying in formation. Everything has tradeoffs. Many things that contribute to lower drag increase the cost, complexity, and build time. Sometimes the relationships between those things run to the square or the cube, and not in the direction you'd want.
My guess is that the mass balance weights are as they are mostly so that people with typical skills can build them from a kit that most enthusiasts can manage to afford. There are certainly other mass-balance schemes that have potentially lower drag, but what Vans uses is a pretty good design. Because of the high Vne that makes the RVs so much fun and so useful, they don't really have the option of not mass-balancing the controls. Doing something heroic like mechanically-linked internal masses (been there done that) adds a bunch of parts that increase the parts count, adds a bunch of pivots and linkages that increase the number of potential failure points, and doesn't subtract all that much drag.
pour your own counterweights! The older RV-4 and -6 kits, pre-1996, were only given instructions on how to go to the local gun shop, buy lead shot, and melt it down for the counterweights. I received my tail kit in early '96, but my written instructions included that information. My building instructor, who built an RV-4 in the '80's, told me how he made his.
