/shrug

SE's never given a reason. They apparently had commands in 1.0 that would allow it, but hotbars & macros & commands all changed enough that it's not clear whether the capability was intentionally removed or if they just didn't want to bother implementing it with the new commands, or if they just haven't gotten around to it yet.

If it's an intentional removal, it's dumb, given that you can work around it by copying hotbars. If it's not intentional, I really wish they'd get on it already.