I believe that this is just an arbitrary choice that doesn't collide with other keys and is fairly consistent with the other keybindings — cs for changing delimiters, ds for deleting delimiters, and ys for creating delimiters.

Thanks! That's really thorny. It goes against the grain of composability and logically destructuring the command into verbs and actions and modifiers.

They should not overload y for yank. Choose anything else that's not used.