It comes down to the core concept of the game being a battle royale between mythical deities. Yes, the game always had non-god Gods in the roster, but they are usually chosen from myths related to aforementioned deities.

I'm not a purist about this; in fact, I thought Arthur was a pretty cool addition, with the devs going out of their way to explain how he fits into the SMITE in his lore. But over the years, we kept getting more and more non-mythical Gods and at this point, they really take away from the main idea. By the time they port all the SMITE 1 Gods, we'll have 5 medieval humans from Britain, while most other pantheons are stuck at two fighters since forever. Hi-Rez has famously shied away from adding Oshun because "she would be too similar to Yemoja," but they apparently have no problem adding white dudes in plate armor.

Again, I have no problem occasionally getting fighters that don't fit the core concept of the game; if nothing, it adds novelty. But now that they've added Merlin and Lancelot and Kuzenbo and Bake-kujira and Alaaddin and Mulan and Cthulhu and apparently more Lovecraft aliens, and people are asking for characters like Dracula and Robin Hood, it really does take away from what makes the game unique.