I always presumed that to be them focusing on pantheons that players would actually regularly use in games.Using real-world pantheons seems to be a trend they're moving away from. From 2e to 3e they've dropped things like the Babylonian, Sumerian, Finnish, Celtic, Indian, Chinese and Japanese pantheons, to just focus on Norse, Greek and Egyptian. 5e starts with the Norse, Greek and Egyptian mentioned occasionally, but Tasha's Cauldron contains no references to any of them.
Egyptian, Norse and Greco-Roman gods have more of a place in our culture, and are myths most players would be more familiar with and likely to want to use in a game.
I've seen people play clerics of Apollo or Thor or Thoth. . .I've never seen anyone play a cleric of Sumerian or Babylonian gods, and the only time I've seen a cleric of any Finnish deity come up in a game was in the context of Mielekki and Loviatar in Forgotten Realms. . . and it's been over 20 years since I've seen someone play a cleric of a Chinese or Japanese deity.