I suspect the problem is that the core coded (which I understand is Delphi (Pascal) based) is rooted in the 16-bit origins of BIAB, which along the way were converted to 32-bit (and even then, there were components that were left at 16-bit initially). From what little research I've done (out of curiosity, not as a programmer), most of the problems with then converting to 64-bit are pointer related, and the "size of" returned values. Embarcadero provides guidelines on converting to 64-bit :
http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Converting_32-bit_Delphi_Applications_to_64-bit_Windowsand I suspect they've probably tried this, but the returned errors are probably massive.
And rewriting from scratch in a different language with all features intact is really going to result a version 1.0 program that will probably be extremely buggy. I remember when Finale went to 64-bit, the posted a number of features that would not be available (at least initially) because of the conversion requirements. I don't know that all the features need to move into a 64-bit version - I mean, how many people are really running "Pitch Invasion" or "Music Replay" on a regular basis?
I want to see a 64-bit version of BIAB (and RealBand) as much as anyone), but I suspect getting there is a heavy lift for a relatively small company.
That being said, I bet there could be a dedicated group of volunteers to test nightly builds of buggy 64-bit software in order to get there.