Because they needed a way to get audio to the AirPods wirelessly and to work with their devices? That’s a pretty good reason to use Bluetooth.
I doubt they got together and tried to scheme a way to break Bluetooth in this one tiny little way for vendor lock in. You can use the basic AirPod features with other Bluetooth devices. It’s just these extended features that were never developed for other platforms.
HN comments lean heavily conspiratorial but I think the obvious explanation is that the devs built and tested it against iPhone and Mac targets and optimized for that. This minor discrepancy wasn’t worked around because it isn’t triggered on Apple platforms and it’s not a target for them.
Some pictures here: https://www.reddit.com/r/assholedesign/comments/b1u08k/this_...
But you can have an extension cord which accepts USB on one end but doesn't accept USB on the other.
So the keyboard has a superset connector so that it can go in regular USB and notched USB, because it is verified to work right when using the extension cord.
This design also means you can't plug one extension cord into another to get an even longer distance (which the keyboard wouldn't expect). Pretty clever solution.