"If we try and make D&D sanitized of all controversial topics, the game cannot exist." That's true. But individual controversial topics should still be something you opt into. They shouldn't be a major focus of a product.
Slavery is a huge trigger. Right up atop the list with colonialism, sexual assault, and child endangerment. It's hard to think of a more divisive issue that carries more baggage.
Dark Sun's a campaign where slavery is major. Slavery's in every single city. It's tied into the backstory of humans and muls and half-giants.
Wizards is always gonna to publish big audience products. Always. Safe products. Middle of the road. Specially now when DnD is bigger than ever. They're not gonna to do weird, niche experimental products anymore. That's for those secondary publishers, like Kobold Press or Monte Cook Games.
Just like if someone at your table is uncomfortable with something you do not include it in your game, if the MAJORITY of the DnD audience is uncomfortable with something they won't publish it in a book. Or do their best to make it optional.
Wizards is trying extra hard to move past its racist origins at the moment and earn some goodwill. They're not gonna endanger that with Dark Sun. That's be like doing Maztika or Kara Tur. Wrong product at this time.
Maybe for 6th Ed. Dark Sun was just released last edition. Seems fair to give a campaign that wasn't updated to 4th Ed a chance instead.