What makes macOS 11 Big Sur supported on a 2013 MacPro and not a 2012?
As to the reason why Apple chose this exact model as the cut-off point for support cannot be answered by anyone but Apple. Everything else is just speculation.
There exists an non-official guide on how to install the Big Sur beta on the older 2010/2012 Mac Pro's (amongst others):
https://parrotgeek.com/bigsur/
Note that because this is non-official, it might not work or exhibit problems. Also it might only work for the current beta, and not when you get to the final release of Big Sur.
As indicated in this guide, the built-in WiFi does not and there could be issues related to sleep mode. This could hint at why Apple does not support this model - i.e. having to support these older WiFi chipsets.