Well, for the setting I'm developing as a D&D setting, Wandering Stars, I'm leaving out halflings for the same reason others are leaving out gnomes-- the two are too similar, and I simply have much better uses for Gnomes. The main races in Wandering Stars are human, elf, dwarf, gnome, (hob)goblin, orc and a scaly race that's about half lizardfolk and half dragonborn. There's no Sorcerer class because draconic bloodlines simply aren't a thing outside of the Sauren, who have them as optional racial features.