If you have different difficulty settings, aside from the work, you also have the problem of the fact that most BAD gamers don't think they're bad so they'll play on Hard, and complain, when they should be playing on Normal or Easy. So then you get developers making the game, calling that Hard and making it easier for Normal (and then still easier for Easy). Blegh.
In addition, Super Metroid has no difficulty setting. :^P
Seeing as how it's more work, having different difficulties falls under the "if we have time, but there are other things I'd rather do" umbrella.
To be fair, Super Metroid doesn't exactly indicates what you have to do, so there is the problem of many "newcomers" who are slightly interested on the metroid series but then, when they play the game, they are confused, most of the time with questions being like "Where do i go?, "What should i do?", yes, you might not believe it, but there are people who got confused at metroid because it lacked a tutorial, of course you could read the manual, but in this age of gamers who don't read shit, they are most likely to fail a game the first time they play them.
So, even if you make the game without difficult settings, you have the problem balancing the difficult throughout the whole game, many people complains about SM being too easy to beat, since this game has a lot of checkpoints, predictable enemy patterns, and while the exploring and the little tricks you could do was really fun, the game itself isn't too hard to beat, a dedicated person could beat it in 4 hours or less, which isn't too much (Hell, i beat DCK2 in 5 hours with 30 minutes, and the first DKC only on 4 hours).
So, the problem with a game without difficult settings is the balancing, since you can't please everyone on a videogame, you either: > Make the game accessible for the "Bad Gamers", and make them feel rewarded, and as for the "Good Gamers" (God i hate the "gamer" term) you make some paths for these kind of gamers and make them feel rewarded while not discouraging "Bad Gamers" to not play the game.
Have you ever played Starfox 64 3D?, well, it's more or less the same philosophy, you have 3 paths in this game, each one of them with a different ending or different enemies, depending on the route you've taken, most players would take the basic predefined route during the beginning, but when they obtain better skills through the game, the second path is available seems more like the preferable option after the first play through with the first path.
You can't please everyone, but you can give options to anyone, "Bad Gamers" tend to obtain better skills while playing the game over and over until they get in a shape form, and when they finally become "Decent/Good" gamers, what are you going to do there?, what are you going to do when they obtain better skills at playing games?, they will simply play harder games and forget about the "Easy" games, so, the point of creating different paths for anyone available to choose while not discouraging bad players to not choose the game.
Now, if a "Bad Gamer" choose the "Hard Path" (Paths and Difficulties aren't the same thing) and they quickly lose, is their fault for losing quickly, because they choose something that is not accustomed to their skills, they must select an easier option while not make them feel bad for playing an "Easy Difficult", which is the previous point i mentioned, creating paths in the game allows a wider range of variety, and therefore, more gamers will be interested to learn the mechanics in each path, so, with that, even hardcore gamers will be interested on each path to see the differences between one, i think this is the best choice you can do.