Probably because we haven't had any sourcebooks for those settings for more than 10 years. Hell, we haven't even covered the entire Forgotten Realms with the modern sourcebooks. We've had them mentioned in the PHB, but no actual detail. They're into franchise crossovers now, Ravnica, Theros, and Strixhaven, and the 6 planeshift worlds are all from Magic: the Gathering. I posted up the Dragon Lance UA in a D&D group when they came out and only the oldest people there even knew that this was a D&D setting. I don't expect anyone who only started playing D&D in the last decade to know what Greyhawk, Dragonlance, or Dark Sun are. The Ravenloft book detailing the domains of dread was only released last year, and it's not very popular, except with people who already knew the liked the setting.
There's also licensing issues. Wizards don't automatically have the creative rights (let alone film rights) to every setting they've ever published. We might have had sourcebooks for those settings sooner if not for the massive legal negotiations regarding ownership and licensing of specific settings that has been going on for many years now. The fact that we are actually getting a Dragonlance adventure at the end of this year means Wizards must have settled with Weiss and Hickman on that matter.
