If I had to guess, i'd say that it was the "fine-tuning" and "completion" parts of it all. They most likely want to ensure that they fix any 'major bugs' as it'd suck to finally release it then get tons of posts about a game-breaking bug early on. Also like MrPhil stated before me.. there could be one or two modders still needing to complete the portion that they are working on.
Personally, i'd prefer they wait and release a polished 1.8 rather than an incomplete and buggy one. If that means it won't come out until 2012 then so be it.
Plus it's already December, and the holidays (Thanksgiving, Christmas (or whatever you celebrate), New Years) raise their heads this time of year. They have lives too.