x86 boots in 16-bit real mode. Then you need to specifically transition into 32-Bit, and from 32-Bit it can be transitioned to 64-Bit Architecture...

The last step (32-bit to 64-bit) can a bit of a can of worms especially on older platforms where 64-bit implementations can differ greatly and 32-bit "just works tm". 32-bit is quite well supported and has enough resources to make some interesting programs work without much hassle.

I think the author has made the decision not to support 64-bit mode due to needing to balance the complexity and usability of the project. It is a hobby project after all.

Since the author maintains a 16-bit and 32-bit for this project I suppose if you wanted you can always fork and maintain a 64-bit version if you wanted to.