Smedberg did post the reasons, with most making sense (via TNW):
Many plugins are not available in 64-bit versions.
The plugins that are available don't work correctly in Firefox because we haven't implemented things like windowproc hooking, which means that hangs are more common.
Crashes submitted by 64-bit users are currently not high priority because we are working on other things.
This is frustrating for users because they feel (and are!) second-class.
It is also frustrating for stability team triage because crash-stats does not easily distinguish between 32-bit and 64-bit builds in the topcrash lists and other reports. We basically ignore a set of nightly "topcrashes" because they are 64-bit only. (See bug 811051).
The engineer did note that 50 percent of all the nightly build testers were using the 64-bit builds, so the new decision will likely anger at least some developers and fans.