IMO, Magic: the Gathering settings don't count as "new settings". Sure, they're new D&D settings (as in new settings to D&D), but that doesn't mean that they're new settings as a whole. Ravnica and Theros both existed before they came to D&D, and they didn't include halflings when released for M:tG, so to just add halflings, dwarves, orcs and other D&D races to the settings when they came to D&D would be changing a large part of the world (it's racial options). Yes, you could mention Strixhaven as possibly being made for both M:tG and D&D in tandem, but that doesn't seem like a valid argument to me, because it's still a M:tG world and meant to be designed that way, and M:tG worlds typically don't have halflings.
WotC is making two completely new D&D 5e settings. I'm willing to bet that if we ever see them, they'll have halflings in them. If they don't, I'm also assuming that they're not just going to leave out halflings and instead do its whole own, new thing and ignore the PHB races (except possibly humans).
That's the difference.