Because the inclusion of slavery in D&D is, or at least has been, a contentious issue around here. And it was so long ago representatives from WotC basically said they wouldn't touch Darksun with a 20 foot pole because of its problematic elements. But then other problematic elements that are hot button issues online have been included in D&D 5th edition products since 2014. I'm not sure what to think. If Dark Sun is on the horizon, I have high hopes for it, but my hopes are tempered by the mixed reception of some of their more recent setting releases. Spelljammer, I'm looking at you.