DMZ2112
Chaotic Looseleaf
I just think it is because someone is a fan of Tieflings. But I agree with you, as the default setting is FR, both the Tieflings and Aasimar should have been included or held until PlaneScape.
Forgotten Realms is not the default setting of D&D5.
I would venture a guess to say they put tieflings in there because they are popular, and they excluded Aasimar because they are not.
This. Aasimar weren't even a particularly great idea when they were introduced to Planescape 20 years ago. Tieflings were in the core boxed set because it made sense to have a race of street urchins running around Sigil who were descended from (relatively frequent) fiendish interference. That same logic never applied to aasimar, who are descended from planar beings who are not intrinsically lustful and who aren't likely to abandon their children.
Still, I think what they ought to have done is made the D&D5 race "planetouched" and provided enough subraces to cover tieflings, aasimar, and genasi, because if you do that you just might pull together enough individuals to consider "planetouched" a category of people. Standing alone the "races" never made any sense, even with D&D4's tacked on backstories.
Sigh. In truth, I regret the D&D5 tiefling and am in no hurry to see other Planescape-derived races show up in the rules. D&D4 savaged what were originally some interesting optional rules to create core material that obeyed the "rule of cool."