Uhm. Maybe because some functionality is not working?
Of course, I know you are right: eventually, it's an end-of-life situation and we are simply forced to use the New Outlook for Mac.
Like I said at the beginning: I am not against updates, upgrades, changes, and improvements. I simply dislike being forced to getting switched to something that doesn't fully work (yet).
I am generally very happy with Microsoft Office. I understand Microsoft needs to streamline apps and services. Over time there will always be situations like this. I am not against that. But not being able to opt out of a switch like this, being fully aware that some features are not working, is very annoying.
Outlook power users that use it across platforms/OS's and are depending on it for daily business and operation should not be confronted with this, until a switch offers at least the same capabilities as today's version.