There's a bit of a difference between SEEMINGLY random moves (since nothing is truly random, if a player does a move, they intend to do that move, to the opponent though, it doesn't seem that way, and it's a strange move in a strange situation, especially something risky that works, we tend to blame it on the random) and mixing up gameplay.
Seemingly random moves is throwing moves out there and just hoping it will hit without really having a good reason. One thing that might cause people wanting to do this is if it works against certain people. To me, i played against a guy who i would use B whip a lot against, and so against other people, i would throw out B whip in "weird spots" but it's just me trying to hit it on the other opponent the same way i hit it against the guy i played against the most.
Mixing up gameplay is like doing a reverse beat into a throw or jab mixup. It gives the opponent a direct 50/50 (for the most part) situation that they are forced to try to stop. Seemingly random moves can generally be stopped by just not attacking or not jumping.
Random moves might work once or twice, or for a match, but really, to level up a lot, you have to not rely on the fact that your opponent is gonna be scared of the random moves. What if they dont fall for your first "random move" attempt and you get hella punished. Now you're scared to pull out the random move with no backup plan. You have to learn how to just make the best out of every situation, and just tossing a move out doesn't always accomplish that. EVERY TIME you throw out a move, you should throw it out with a clear goal in mind and some reasoning behind it. A random move might not be bad in the case of like: "everytime my opponent does a 2C 5A they run up and throw afterwards if blocked. So after they do 2C and i see them whiff the 5A, i'll throw out a dragon punch, which would seem random to them, as it's not on wakeup, and im not coming from the air." If it hits, the opponent might check it off to randomness. The thing is, do that against the next guy you play, if it works, they'll be like "huh, so random, why would you do that there" etc.
Its like in this sausagefest2 vid:
http://youtube.com/watch?v=oz6o6U0QZFsZar was commenting on the match. I do 2C 214A(with Sion of course) and then Spooky blocks it and i DP from like, quarter screen away, further than DP distance, thats for sure. It hits and Zar goes "wow, random DP into 623C...or maybe it wasn't random, completely intentional"
Obviously, it was a small setup i knew, but at the time, it just seemed like i was tossing stuff out for the sake of it. Not that i plan on scaring my opponent with that tactic, it was just a "use once and hope they do what i want them to do in that situation" thing. So im not saying don't use setups and stuff, but don't base your gameplay off of hoping stuff that you just kinda toss out hits. Don't ever rely on one of those V.Akiha 2BB's for the kill because the second you start to do that, a good player will block it and kill you for it, and thats the match.
As for the US Melty scene. Simply put, not enough people. As for the lack of characters thing, I know it's gonna sound mean, but just HAVING someone who plays a certain character doesn't always mean much. If there's a guy who uses, say, Warc, but he's really not that great, then the community, while it has a "Warc player," it doesn't mean that the person is any good. If someone just isn't that great, they can only help the scene so much. This is why Zar is referenced so much. He is REALLY REALLY good. Its not who he uses, its how good he is. He puts lots of time into the game, studies it etc. Does it matter if he uses Aoko or Akiha?
Not really, if the top player is the top player, then whoever they use is gonna be who people find the toughest. Regardless of whether its a character "everyone uses" like the 5 you listed, or a character that not a lot of people use like Len, Warc, Miyako, etc. What matters more is how good the players are.
If Arly was winning with Kohaku instead of Nero, would it make a difference? No, what matters is that he's a top US player, and because of that, the players around him can get better.
It doesn't matter if you're the only one who uses your character in the US, or if you're one of 15 people. What matters is your skill level.