To cent here
Apple has not stated that Intel Macs were excluded because of any single technical limitation. However, Apple has documented that macOS 27 Golden Gate is designed for Macs with Apple silicon, while macOS Tahoe 26 is the final major version of macOS that supports Intel-based Macs.
This follows Apple's transition to Apple silicon, which began in 2020. Apple has been moving its software and developer technologies to the Apple-silicon architecture during that transition. Apple also recently confirmed that macOS 27 is the final release to support Rosetta, the technology that allows many Intel-based Mac apps to run on Apple-silicon Macs.
Therefore, the most accurate explanation is that macOS 27 marks the end of Apple's transition-era support for Intel-based Macs and moves the Mac platform forward as an Apple-silicon platform. It should not be interpreted as meaning that every Intel Mac is suddenly unusable. Intel Macs that are compatible with macOS Tahoe can continue to run that version of macOS and receive applicable updates from Apple.
Apple's macOS Tahoe compatibility list also shows that several Intel models—including the 2019 MacBook Pro, 2020 Intel MacBook Pro, 2020 27-inch iMac, and 2019 Mac Pro—were supported through the final Intel-compatible macOS release. (Apple Support)
I would recommend not saying that Apple excluded Intel Macs because they are "too old," "too slow," or incapable of running the new features unless Apple specifically documents one of those reasons. The documented facts support the broader explanation that Apple's Intel-to-Apple-silicon transition has reached its endpoint with macOS 27. (Apple Developer)
Useful Apple documentation: macOS 27 Golden Gate, macOS Tahoe compatibility, and Apple Developer information on the Rosetta transition.