For the lighter and faster desktops in Mint 9, only Xfce will see a 64 bit edition. At the moment the people who are primarily using Fluxbox and LXDE are using older hardware setups, most of which will not support 64 bit. This isn't to say that there aren't people with 64 bit capable machines who are, but just that that particular segment of the user base isn't really that large, especially considering that the Fluxbox edition covers a very small niche market in the first place.
Also, there's the aspect of the amount of work that goes into it. Basically, most of the legwork can be done with the 32 bit edition and the same changes applied to the 64 bit base, but then there's testing and debugging which can often severely vary between the two. I did one preliminary 64 bit build of the LXDE edition fairly early on, but the executive decision was to wait and reexamine the option for a later release.
Also consider that Fluxbox isn't exactly a well maintained project anymore. Only very recently has there been any new activity on the development front with it and I'm unsure as to how that will progress in the coming months. That, and there are some bugs in it that I don't foresee ever getting fixed. This poses a question as to the future of the Fluxbox edition. Personally I'm a bit of a Fluxbox junkie and have used and preferred it for almost as long as I've messed with Linux in any capacity, but even I'm finding it hard to put a lot of faith into the continued development and improvement of the project
It's true that I'm busy as I'm now producing 4 releases for 2 distros on top of a job that demands overtime every week. I'm leaving my job in October for a sysadmin position that will free up a very large chunk of my time, so I'll have much more time to invest in Linux related projects. We'll see what the future holds, but right now Mint 9 Fluxbox is only going to be a 32 bit release and I'm (hopefully) very close to having the release candidate out.