The actual problem is one of power and intention. Yes, version control would make earmarks obvious. Yes, it would make tracking contributors (lobbies) easier. Yes, it would make tailoring tried and trusted legal documents easy.
All of these are reasons why version control will never be applied to the law. We want these things, but we are not the customer. The real customer is actively trying to prevent these things from ever happening.
The reality is that these tools are probably already being used for these exact purposes - in private, and for personal gain.
However, in the 200+ year history of the United States Congress, no tool like this has existed that could bring the kind of transparency to the process that reformers have been pushing for for nearly as long.
It is one thing to say "We need more transparency!" It is another thing entirely to say "Publish all bill modifications with attribution using a public version control system!" Having something specific and concrete to push for, something the implementation of which can be said to be completed or not completed in definitive terms, can make the difference between a successful push for reform and an unsuccessful one.