S
Sunseeker
Guest
Or they could be leaving it up to the DM to decide if XYZ race exists in their home games.
IMHO, the PHB was not designed with the home-game in mind. Its setting is clearly set. It's lore is prominent and pervasive. The "uncommon" tag was added, again IMHO, to address the fact that the lore that is so interwoven with the traditional races (and pretty much all of 5E) doesn't address the existence of dragonborn, tieflings or other uncommon races well, or at all.
The DM can ALWAYS control what races are, or are not in their games. The book doesn't need to tell you that X race is common, Y race is uncommon and Z race is rare. It's a home game! By the nature of it being a home game, NONE of those things apply. How common any given race is is entirely at the discretion of the DM. Just as whether or not the race in question is playable.
Adding the "uncommon" tag doesn't affect the level of DM control over their own games. It affects the perception of players. Honestly I dislike when the book says "consult your DM about this, that or whatever." I feel like it drifts into the realms of turning a cooperative gaming experience into a directed gaming experience.