The "preferred form for modifying the work" has changed with time as the state of the art has developed. In the 1980s and 1990s, there was sccs, rcs, and eventually cvs. Most development was done by small teams, and the growth of larger free software communities, and a social environment accompanied by effective source management tools to support that social environment -- was in the future. Did the FSF lag behind some other organizations in moving to these tools? Yes and no. Though some of their projects, which are often quite "loosely" within the FSF umbrella, moved faster than others, the FSF also promoted early DVCS efforts like GNU Arch, and made sure that alternatives to the early proprietary social programming sites like sourceforge (which was briefly free software) were available.
We owe them our gratitude. Remember that their resources are often orders of magnitude less than that of even the free software companies.