When you're preparing a game for your own group, you know who the characters are: Fred is playing the druid with the dire sloth, and Charley still has that annoying ring of invisibility. You know what kind of challenges would suit them and what wouldn't.
But when you're designing adventures for publication, you don't have that luxury — you never know who the player characters are going to be.
Unfortunately this encourages publishing very middle-of-the-road adventures. It makes sense: you want to write adventures that the most people will be able to use. The more exotic the requirements, the less likely the average group will have any use for your scenario. You might come up with a very cool plot idea that requires one of the player characters to have a familiar (bats and black cats gone wild!), but since you can't assume anyone will have one that idea gets put on the back burner.
You can assume the heroes can fight, because combat is a mandatory element in just about every mainstream roleplaying game, so include all the fighting you want. But any section that requires special abilities or traits may not work for some groups. Want a prolonged sneak past guards and wards to get to the inner sanctum? Sorry, nothing but platemail in this party. Want a horse racing sequence? Dwarves don't horse race.
Preparing a game for unknown characters is tricky in any game system, but it's doubly true in a superhero game, because superhero characters can turn out to be just about anything. One might be a flying strong guy (easy), one might be an electrical zapper (easy), and one might be an incorporeal alien thought-form who can walk thru walls and read minds (not so easy).
Partially it's because most supers games are point-buy rather than class-based games — except for equipment and a limited choice of spells, you can predict what a bunch of 10th level D&D characters will look like. There's just not that much latitude. Not so in a game where you can buy whatever you want from the ground up.
Mostly it's a matter of genre. Even if you run a point-buy modern spy game, the genre doesn't allow you to make a guy that can walk through walls or read minds, no matter how many points you get. In superhero games, almost any concept fits in somewhere. Cowboys, aliens, teleporters and ghosts: they all happily co-exist in superhero games.
Of course there's the flip side, and that's when the published adventure does require certain types of player characters to work, but fails to recognize it or warn the GM.
Sometimes its obvious, and the GM can spot it before the game starts. Other times it can be far more subtle: an adventure might not _require_ a bunch of diplomatic characters, but it will work much better the more there are. Note, it's not just “the party will have an easier time” if they have more X, but “the game will play better/be more interesting/etc” if they have more X. Big difference.