This.
Nintendo wants to sell the DLC characters to the players, not to those within their staff or a minority in the community that think "oh this character's origin game was important for gaming history, I will buy him because of that reason".
What do players care about characters? Either a) they love the character because it's a cool concept, personality, or whatever honest reason players these days have for wanting a specific character, b) they simply love the game it comes from, c) they love both aspects mentioned before. None of these reasons is because of "what the game did to gaming history as legacy". For example, people are gonna buy Joker because Persona 5 is amazing game and the character is cool and blahblahblah, NOT because it did anything special to gaming at some point in history. So it can be said about ANYONE's favorite here, either already in the roster or yet to be hopefully added as DLC. They will buy the character because they ACTUALLY WANT HIM/HER from a personal perspective, not due to history importance.
If Nintendo wants to add a character that is important to gaming history and not be afraid to lose money on it regardless of the game's current situation (such as DQ in other regions/countries that aren't Japan), they add such character to the BASE ROSTER, they don't risk it for DLC, where people will only spend money what they actually want. Sure, DQ has sold a lot in japan, and that might ensure almost 100% of players there will buy a DQ character, but it's never a big win if only such DLC character will sell well in their home land and still badly on other regions/countries ( 2,3,4,5,6 etc > 1, so it doesn't matter if it sells well in japan, there will be a loss if it doesn't sell well EVERYWHERE).
I have nothing against Eldrick, nor DQ, but let's be real here already and stop giving "legacy in gaming history" such a strong point for why a character is a good idea for DLC, cause it's not the honest reason why we all players buy a DLC character in the end, we just don't buy a character simply because of THAT. Me personally, I'm buying all characters because i like to have a full game to play with, it's always been that for me, and that's an honest reason i can see many other players will have for actually buying Erdrick, NOT because DQ was such a big deal to the genre in gaming, lol. And this can be applied to any franchise from any other company, it's not a personal bias against DQ or SE or whatever, we're actually being objective here.
A decision that makes sense in sales would be a character that sells well in more than one region. It's dumb to just make a character DLC to sell well just in japan and do terrible in the rest of places. You want DLC to sell for a game worldwide? Make worldwide choices. We can recognize all we want that DQ sells well in Japan, that still won't change the reality of sales in paper when they see that a DQ character only sold well in japan. If they want to make money the best way possible, the best take is to focus on worldwide picks, not a specific region's picks. Edit: Joker is a good example of this, a franchise/game (persona 5) that is beloved everywhere, hence why the reaction worldwide was amazing and people feel likely to buy the season pass because of him. You wouldn't see this happen with a character that is only highly beloved in one region and minimally beloved everywhere else.