Sometimes, projects don't have sufficient developer help to properly maintain two separate branches, so they decide to put all their concentration and effort into one branch, or the other, rather than attempt a mediocre stab at both. I'm pretty sure we'll be seeing more of this happening with 32-bit vs 64-bit as we move forward, especially with 64-bit being predominant.
While I still have 32-bit hardware my preference is for a distro which supports 32-bit. At the moment there are still plenty of distros supporting 32-bit (and 64-bit) systems, including the ones I use. If the distros I use decide to drop 32-bit support while I still have 32-bit hardware I will have little choice but to switch. Dropping 32-bit support excludes some people from using a distro ... unless they change the hardware they have. I have a choice to run 64-bit software on the hardware I have which is capable of doing so ... and I do - but I would not run a 64-bit only distro on it. If I had only 64-bit hardware my opinion might be different ...