Seen a few people ask why race rather than class provides ability bonuses – ideas in D&D 5 was to make pathing as easy as possible for new players. Races are meant to be very accessible/pop culture driven. They then bridge into class via stat bonus, making that choice easier.
— Mike Mearls (@mikemearls) September 25, 2018
Sub-races serve two purposes – give races more flexibility without adding too much conceptual complexity. They also are a convenient place to plug in elements needed for settings – I adore how @HellcowKeith used them for dragonmarks in Wayfarers Guide to Eberron.
— Mike Mearls (@mikemearls) September 25, 2018