If it makes you feel any better, I get tired of the "good/bright things are boring, bad/dark things are awesome" thing as well. I find that sort of thinking extremely tedious. Darkness is only enriching when it is adding contrast and nuance. Piling darkness on darkness on darkness on darkness is exactly the opposite of interesting in most cases. It's nothing new.
That said, I think the answer is a little more complicated than some of the other stuff already said. It's not "angst" per se--it's that tieflings are necessarily going to have
more conflict than aasimar, all else being equal. Having angelic ancestors is, to use a term I've recently heard, almost entirely
"candy" with no "spinach". (I will be
heavily drawing on this essay for this post, so if you prefer, just read it from the source.) It's awesome, and laudable, and positive. You can still wrangle a conflict out of that--unearned valor, or impossible legacies, or being crushed under the weight of others' expectations--but it's necessarily a less impressive conflict because you get unobjectionable, positive associations out of it. ("Candy" is stuff that
glorifies a character; "spinach"
humbles a character. It's possible to go too far in either direction, but excess candy is much more likely than excess spinach, generally speaking.)
In that sense, an aasimar character is sort of like an otherwise-normal person who is a megagenius, or an otherwise-normal person who is super attractive and charismatic. (And, indeed, many aasimar characters ARE that second thing.) It's all benefit, no downside; it's all candy, no spinach, and even if you work some spinach into it, it's still heavily biased toward candy.
Now, being a tiefling
isn't devoid of candy. But it's more complicated. Having awesome devil horns or bright-red skin makes a character special and distinctive and perhaps intimidating (all forms of candy), but it also makes the character likely to be hated, outcast, or hurt by others (all forms of spinach, just
lesser forms of spinach). There's really no spinach that accompanies the characteristics an aasimar would evince.
In general, people need to see a character get a sufficient amount of spinach early on in order to feel that it is worthwhile to invest interest into that character. A character that is sickeningly over-candied will drive audiences away because it looks like nothing more than a trite fantasy with nothing underneath the surface. But, once an audience
is invested in a character, they almost always want that character to get much more candy than spinach. This results in a frustrating dichotomy: if you want to get more people on board with the character, you give them spinach-y traits or problems....but in so doing you may
drive away some of the people who were already on board.
There are of course exceptions to this. Sherlock Holmes is an almost exclusively candied character, but he's still interesting because we want to see him be smart and solve mysteries, and those mysteries are usually compelling enough to keep our attention. A similar thing applies to James Bond; he's an EXTREMELY candied character, but we accept that because it's fun to watch him take on equally-candied
villains and discover where that will go. But,
generally speaking, people are going to look at character elements with an eye toward a starting balance that slightly favors spinach, but slowly grows to favor candy (often, to
heavily favor candy).
Aasimar is too much candy, not enough spinach. Tiefling is still a lot of candy, and some folks may not find it compelling as a result. But for a lot of people it's a much healthier balance between the two. The spinach of social problems and the dread curse that might linger in your blood and (etc.) provide some counterbalance to the impressive aesthetics, magical powers, and imposing nature that come from fiendish ancestry.
