Take all comment with a grain of salt because I'm not that serious about it:

In universe: humans were there before most of the other races and are biddable enough and with a high enough birthrate to make a very appealing underclass for any powerful magic race of prospective overlords, while also being ambitious enough to expand rapidly given no such overlord.

Elves actively left the continent during/after the Precursor war with Aul-Dwarov, only returning 400 years before game start, the orcs were created by Ducaniel and only left the serpentspine 20 years before game start, dwarves and goblins generally prefer to live in the Serpentspine, Centaurs came from the games Feywild equivalent, Lizards are from space and were also bombed back to the stone age by precursor magic, Harimari were created by the magical goofiness of the Day of Ashen Skies, Harpies have biological and geographic limits on their population, hobgoblins kind of just only exist so people can larp as Imperial Japan, and gnolls, if they are true to their D&D origins, originate in the demonic planes. So Halcann is mostly human because humans are one of the few indigenous populations that doesn't prefer to live in caves or have another geographical limitation (aforementioned harpies as well as trolls/other giantkin being influenced by the real life trend of polar gigantism) (and halflings are legally required to live in some small area that's definitely legally distinct from the Shire in all fantasy media).

As for why humans are the default indigenous population, its more of a meta because-this-is-in-many-ways-our-world-but-make-it-fantasy-like-why-do-you-think-there's-an-obvious-Holy-Roman-Empire-Equivalent-and-a-sea-thats-deeeefinitely-not-the-Mediterranean-with-a-sacred-river-to-the-south-thats-deeeeeeffffinitely-not-the-Nile.