I think I have a theory on why Erdrick and Solo don't have their female counterparts. I believe it's because since the Luminary and Eight don't have female counterparts of their own, Sakurai and the team might have felt it would've be unfair to fans of DQ 8 and 11, since they wouldn't have the same options as fans of 3 and 4. This also could be paired the fact that the alts would be lopsided, since half of the hero has two recolors, while the other half has full new models.
They can also just make none of them recolors and fill in the remaining 2 slots with more protagonists. Pikachu proves that the Smash team doesn't mind making less than half of the color slots female, but it would make more sense on The Hero. After stewing on it for a bit, here's how I'd replace the recolors:
Dragon Quest 1 Hero (Descendant of Erdrick)
Dragon Quest 3 Female Hero (Female Erdrick)
Dragon Quest 4 Female Hero (Sofia)
Dragon Quest 6 Hero (Rek)
Aside from the female alts of the DQ3 and DQ4 heroes, the protagonist of the first and sixth games felt the most fitting to include to round out the color slots. The first game had a hero that could use magic, just like all the current playable heroes, while the sixth game introduced the modern vocation system of class changing, which would go on to be used and tweaked in DQ7 and DQ9 (dunno about DQ10). Every other hero from the series has a good reason to not be playable (2 can't use magic, 5 is more of a monster tamer and isn't the actual Hero class character, 7 is too short, and both 9 and 10 are completely customized by players despite having official art).
Only two of the eight slots go to female heroes, but this way it isn't awkward when it comes to giving certain heroes recolors and others gender swaps, which I think makes up for the gender swaps being an unequal number. The only problem is that this is much more work than recolors, and modeling 4 completely unique models for a single character (The Hero) was already going the extra mile.
All that said, I highly doubt that the Smash team is gonna remove the color swaps for each of the four playable Heroes. So to get the female DQ3 and DQ4 heroes playable, I'm hoping for some kind of scenario where we either get some kind of Echo Fighter called The Heroine, or color slots are expanded to something like 12 or 16 rather than 8 for certain characters / across the board to fit them in somehow. Neither possibility is likely at this point, but I'm hoping their absence is addressed at a later date because it really sucks not having them in the game.