Or it could just be that "this is the way we've always done it" / that Slack has enough inertia in the organisation that changing workflow, updating shortcuts,

Or it could just be that "this is the way we've always done it" / that Slack has enough inertia in the organisation that changing workflow, updating shortcuts, etc. is inconvenient and thus left until the last minute to change over. I haven't used slack but broadly speaking Teams / Zoom / all of the other platforms I've used have been roughly the same, in that they all get the job of text and (usually) video communications done. Some might be a bit nicer to use than others but largely it doesn't matter which one you use as long as everyone is on the same platform.