On the other hand, their two-bit flash player is quite widely distributed.
To me, that indicates a dependency problem. Specifically, a dependency isn't available yet for 64 bit.
I would guess it's because of the JIT compiler to generate machine code (see the Tamarin project in firefox). Writing a compiler for many different platforms requires a significant amount of work. I can understand that they'd want to work on other platforms before the 64 bit processors, since those can still use the 32 bit version.