I was an attendee at / member of that Board a few times in my career. It was a unique construct that had a tendency to be periodically ignored / undervalued by both sides, and was often performative. Nonetheless, these kinds of linkages can go a long way to lubricating the gears of binational relations, and so it is sad to see it go.
As to why? This seems aligned with American Isolationism as a trend / ne policy thrust, and is in keeping with the foundational aspects of Project 2025.
I reckon NORAD will be next.