My guess: The binary translation support they’re going to build into ARM Macs will exclusively support x64, including low-cost bridging between x64 application code and ARM64 library code.
Not to mention Apple never leaves deprecated runtimes around for more than a few years. Once 64-bit Carbon was canned the writing was on the wall.
Arm architecture is rock-solid. The trick is making an ARM processor fast enough for desktop or laptop use. ARM has unfortunately been relegated to low-end devices even though the architecture could do well for high-end machines. We don't have a good interop for running x_86 or x_64 code on ARM so the compatibility isn't there for most users (like Windows 8 ARM). Linux is mostly fine on ARM if you can compile from source and you have a compiler for that given language that works for ARM.
Not to mention Apple never leaves deprecated runtimes around for more than a few years. Once 64-bit Carbon was canned the writing was on the wall.