My colleague Elizabeth Judge has written a really good piece that traces some of the history around this.
Initially, I think some of the concerns were to ensure that a government document could be relied upon, that it was credible and authoritative. I think that is far less of an issue today than it once was.
I also think that the kinds of possibilities we had to use government works didn't exist in the early days of this in the way that it does today. We think of the development of GPS services or other kinds of services built on open government or government data. The idea that we would continue to have a copyright provision that would restrict that seems anathema to the vision of a law that has adapted to the current technological environment.