Because people treated Microsoft like a traditional corporation, while Zuckerman and others of his ilk have convinced those in power that Facebook is somehow above and beyond the reach of a traditional corporate entity, which we all know isn't true.
When I used to get those opinion questionnaires and panels, I got asked whether tech companies should be broken up, I would always answer "yes" and I always made sure to give a very thorough, specific reply on why I believed the likes of Amazon, Facebook, Verizon, etc. should be broken up and not allowed to have all these mergers, acquisitions and various holdings and how I believed they misused all that power to bar competition and stifle innovation and a fair marketplace.
Europe has been leading the way in the antitrust fight, as they,'ve had Google in litigation for the better part of a decade now.
The Antitrust Division of the DOJ was gutted under Trump and is still being staffed up. It still doesn't have a confirmed head. In fact Republicans have made a point of slow-walking all of the hearings for DOJ nominees.
People have to stop thinking of Merrick Garland like he's Dumbledore. The DOJ was purposely torn to shreds and, frankly, it's amazing that Garland has been able to do what he's done in the time he has.
Also, breaking up Facebook will need to be a global effort. FB's corruption is a global phenomenon. Australia seems to be stepping up to the plate: https://gizmodo.com/facebook-users-liable-for-all-comments-under-their-post-1847633797. Other countries will need to follow.