Probably a few reasons, but the one they give in the video is narrative.
Creature type tells a deliberate story in this version of the game. Something about where your magic comes from or why you are the way you are. So if you say your character is a humanoid, that means something specific. It sounds like it means something like "they are basically like humans, with maybe a few little tweaks." So to say that a goblin or a githzerai is humanoid is kind of to say, "this creature is from the material world and doesn't have more than a shadow of inherent magic and raises families and eats food and survives much like a real world human does. Not exactly the same, but close enough that the difference is a little subtle."
In comparison, you have your fey goblins, which are of the feywild, inherently magical in a big way, capable of inherent trickery and illusion that humanoid goblins aren't capable of without studying magic (say by becoming an illusionist). They do not live lives like people do. They dwell in the graces of the courts of the Feywild, where time flows different and the stars come down to play.
I'd also wager that, mechanically, it means "You can use our 'generic NPC' stat blocks for those, with an extra trait or two. A humanoid goblin knight isn't much different from a human knight or a dwarven knight - not in a way that would impact CR or anything. Add a quirk or two from the species traits and run with it." In comparison, a fey goblin is its own thing. Not a knight or a guard or a commoner or whatever, but a creature whose magic defines its capabilities more strongly.
I can imagine a fey goblin that's still balanced as a PC species, but it might have a different set of abilities than the humanoid goblin that's already in the game - more magical, more dramatic. Maybe (and this is speculation) more powerful because it also eats up your Background or something.
