Because then you'd also have to get rid of all those other races that people don't use?
But seriously, does it actually hurt you that they're in the PH? If it's just that you want other, different races in the main book, then why not advocate for larger page counts with a larger chapter for races instead of getting rid of something that 5% of people like when you can and apparently have already ban it at your table.
@Faolyn, things would go a lot more smoothly if you would stop making assumptions about other people's games.
Ban halflings? Why would I do that? I have repeatedly stated that no one at my table plays them or, at the very least, it's once in a blue moon that they get played. So, why would you claim that I've banned something? What are you trying to prove?
Ok, we get rid of the other races that people don't use. Great. What's the problem? Since so few people are using them, it's not really going to bother a lot of people if they get moved to the monster manual. And, hey look, we can add races in that hopefully gain more traction, like tieflings and dragonborn already have. What's the point of having options in the PHB that are so unused?
Again, it's not about liking or disliking halflings. Or gnomes. Or half orcs. Heck, at some point of time or another, I've played all three. Why would I dislike something that is so unimportant? It's like the old giant table of polearms in 1e. Sure, there are those out there that want an expanded equipment list. But, guess what, they lost that argument. Every edition of the game has truncated the weapon list because so many of these things were just taking up space.
So, why would I want an expanded PHB? So we can cart around the cruft that has accumulated in the game to make some very small minority of gamers happy? Don't care. It's not making the grade, so it gets cut. Anything that doesn't make the grade SHOULD get cut. Or, at least moved around a bit so that it can make room for stuff that is making the grade.