To compile a native 64-bit viewer, you'd have to get 64-bit versions of the required libraries in place. For Windows, 32-bit library binaries are downloaded automatically during the build. Replacing them locally by their 64-bit counterparts might be a non-trivial task.
I also guess the Windows-specific parts of the source code might not work correctly when compiled for 64 bit, as they were only tested for 32 bit builds, yet. (Some code might even explicitly assume 32-bit specific conventions.)