I think that if people are willing to argue over it for 60 pages, it is pretty obvious that both those who love halflings and those who dislike halfling lore DO care.
Specifically, a couple of reasons for why this matters have been advanced:
- new settings tend to incorporate the core races, but are less strict about incorporating Volo or MToF races. This might be an issue for players or DMs who want to play in those settings;
- races in the PHB are the only ones that are available for free on many digital tool sets: this also skews evaluation of their popularity;
I would add a third item:
- other D&D products do not use the full complement of races, but tend to include the core races. For instance, despite tieflings, genasi and dragonborn being more popular than halflings, Betrayal at Baldur’s Gate only has humans, elves, dwarves, halflings and half-orcs as playable characters.