Because during the initial Dev phase this very question would have come up and there very likely is a test build set that runs, or tries to run, on the pre-M1 iPad Pro.
I'm pretty sure this is something they decided before they programmed one line of code.
They don't have to prove that it won't work.
Usually decisions like these are very simple to do:
Manager: Can these feature be implanted on older iPads?
Developer: Maybe, but it would require a lot of engineering resources and we're pretty sure it's going to be problematic due to little memory on older iPads. We're not sure we can guarantee a fast experience.
Manager: Are there are any other reasons to not support older iPads?
Developer: Yes, M1 has faster IO to both storage for swapping which can delay slowdown if running low on memory and communicating with the external screen
Manager: How about testing?
Developer: We can save a lot of testing resources and time by not having to test old iPads at all
Manager: Sounds good. I'm going to recommend we drop old iPads for this feature
Upper level manager: You make good technical arguments, but how about sales?
Manager: Worst case, they will still use their old iPad and pay for apps and services where we get a cut, best case: people will buy a new expensive iPad.
Upper level manager: Approved!