A couple of things:
1. Assume, for sake of the argument, that the length of the PHB is fixed. Due to the cost and logistics of printing a hardcover book, we can't just add pages to the PHB. What comes out?
2. If a Race is in the PHB, there's the assumption that it is playable in every campaign setting, and I don't think that's true of Goblins. Eberron had to make some tweaks in 4e and 5e to give space to Tieflings and Dragonborn. I don't see Goblin PCs in Forgotten Realms or Greyhawk.
3. Orcs were also reprinted, but were changed to accommodate Eberron and their history in Eberron. If they'd been in the PHB, that'd still be duplication, because now you're rewriting a core race.
1. Effiencies can be found, you cut back a here, a page there, and so on. It's 10 to 12 pages at most and reject the premise that the length of the PHB was fixed, because the Monster Manual wasn't, it was bigger then planned.
2. This is a common Fallacy, Half Orcs and Gnomes are not found on Darksun or many parts of Ravenloft, Dragonborn aren't in Greyhawk (even Tieflings are iffy), Dragonborn and Tieflings and few other races are in Birthright, Ravnica has no Dwarves, Halflings, Half Orcs, Tieflings, Dragonborn, or Gnomes, ect..., and if we count MtG settings future D&D settings this gets more pronounced, Lowyn/Shadowmoor has no humans, Theros has no Elves, Dwarves, Halflings, Tieflings, Dragonborn, Half Orcs, Gnomes, Half Elves, Innistrad really only has human, Amonkhet is more diverse in playable races, but the only PHB race is human. I could continue, but you get my point.
3. Literally the only mechanical difference was getting rid of the negative penalty, otherwise they are identical stat wise. And races can have different flavors in different settings, but the stats are the same.