It's a pain to redo work that's already been done. If only Ars Digita would incorporate postgres into their dev tree with some ifdefs for when they build the distro package. The community could maintain the postgres version, and they could maintain the oracle version. That way the community would never be out functionality.
Somehow, though, I doubt that will happen.
The difference is that they've got a team of paid programmers working on the software all the time. ACS/pg has volunteers. Volunteers can do great things (Linux, etc), but there's a strength in paid programmers, especially when they're churning out greatly enhanced new releases once a month.
A complete fork would be sad, IMHO.
If we look at the past 'n' versions of the ACS, how much do existing modules change in functionality? If they don't change, or change little, then a 'diff' repository of some sort would work on new ACS releases, and the only porting work would involve new or in-development modules.