Let‘s put it this way: Legacy Software, Office format problems, lack of user training and a heck of money from Microsoft…
And good desktop management tools -- SCCM is a beast to set up but it's extremely powerful and there is nothing, even from Apple, as comprehensive and mature as AD.
It's not really surprising that a 100b company who specializes in writing software for specific business office-ish use-cases can beat an ad-hoc group of corporate volunteers mostly focused on the developer experience and the server space.
I'm curious to hear why the BSI migrated back to Windows, but didn't find anything with a quick search. Any more information on this?