I think a big part of the equation is that D&D is more like Star Wars now. "Monster" races no longer exist in the minds of many players. They're just races that are now spreading everywhere in the world of Faerun, so everyone knows every race now. Again, I'm not saying that this is true from a FR lore perspective. I'm saying that it's true from a what players seem to think perspective.
That's how it went with Star Wars too. Wookies, for example, were supposed to be a rather uncommon race originally. Chewy became so popular that suddenly Wookies became one of the most common races in the galaxy.
The true lore of Faerun is humans are everywhere. Elves, dwarves, gnomes, halflings are also everywhere. Common monster races are everywhere, like goblins, orcs, ogres, trolls. Most other races? Uncommon at best. Tieflings, dragonborn, Yuan-ti, Gith, Drow, etc. They are all more uncommon to the general populaces.
But not anymore. Like Wookies, they are now EVERYWHERE.
