Adoption is the problem. The US Code already is on GitHub —
https://github.com/divegeek/uscode — but it's only a mirror. There's also the Sunlight Foundation's OpenCongress
http://www.opencongress.org/ which provides a nice way to track a bill and its participants. But without the authors actually using something like git in their process of writing the laws, this doesn't really help people participate directly.