I don't. Carbon was a >20-year-old codebase which had always ran on a 32-bit system. I wouldn't be surprised at all if they simply determined that there was too much code which would have to be rewritten to run correctly on 64-bit.
20 years is nothing (/grabs your lips and makes them say "NSObject"). Don't talk to me about code quality either, just because something is old doesn't mean it's bad (or good) it just means it's old.
This is especially true for "the last 20 years" in particular (that featured elimination of most American tech workers with more than 20 years experience during the dot-com crash followed by a six year timeout where upon most American universities did not graduate many CS majors and those that did had no real "industry mentoring", just Google searches and blog sites.) IN MOST CASES, THE QUALITY OF THE OLD CODE FROM BEFORE THAT ERA IS GOING TO BE FAR SUPERIOR TO THE CODE PRODUCED DURING THAT PERIOD.
Things are just now (14 years after the new CS majors began to arrive at their desks) starting..and I do mean starting..to get back to normal (the people who infested the industry in the absence of such formally trained talent are still around, many have whipped investors into FOMO, allowing them to live very well and thus be influential and even exploitative of the younger formally trained CS majors, instead of the other way around).
I'm treading extremely old ground, but fwiw:
I don't fault Apple for pulling 64 bit Carbon from Leopard. It makes sense—the OS had already been delayed and they had to cut stuff in order to ship.
But I find it totally bizarre that they never shipped 64 bit Carbon. Why wasn't it in Snow Leopard? Heck, Lion even.
Catalina's nixing of 32 bit would be a lot less painful if Carbon apps had some kind of upgrade path.