In an online FPS, for instance, a casual player can still occasionally kill an experienced player, and the number of players in a given game and the nature of quickly respawning after you die mitigate the effect of skill imbalances and allow the game to be playable for those who aren't in the top-tier of players. In addition, the learning curve is generally shallower and the number of effective playstyles higher.
You have a good point. In some FPS it's possible to contribute by "hustling"--by being a hard-working player. Unreal Tournament's (1999) Domination game is a great example, and so is Onslaught from UT 2004. When you respawn you hustle right back into the action, throwing yourself at a small goal relentlessly.
In an online RTS, on the other hand, the involved nature of each game makes it very frustrating for a casual player to play against people who are better than he is.
One of Sins's big problems is a lack of skill-based pairing. It wouldn't make much of a difference right now since the player counts aren't high enough to sustain something like that. One improvised way around this is to play 5v5s and have two captains draft the teams from a pool of the other 8 players, which is a way of trying to apportion skill equally across the two teams.
Generally RTS games tend to fall into the pitfall of being fairly limited in the number of possible winning strategies - one must learn one of a few strategies to be competitive, usually by watching replays and observing what the more experienced players do. This, combined with the amount of time that you must devote to a single game (especially for SINS), makes it a nightmare for a casual player.
Sins is pretty dynamic and there are numerous fleet builds, tactics, and strategies that a player might use. The online multiplayer team game is much deeper than single player against AI in those regards. The AI is very forgiving so it's not nearly as important.
Aside from micromanaging units in combat, the strategy in this game is all about balancing economic development, military needs, and research. I think you need to get a sense of when you need to "peak" with your fleet. When you need to get as many ships as possible so that you can invest to attain that. Scouting and assessing the enemy and preparing to counter him is also very important. Then you need to decide when and where to attack or where to defend.
Aside from requiring tactics and strategy, the multiplayer team game also introduces team-play-based elements into the game. Sometimes you'll have to do things to help an ally, even if they don't ask for it, so you might need to keep an eye on what's going on with certain allies next to you or on the other side of the map. You could feed them credits and resources if you are in an eco slot, build starbases at their homes if need be, or have your fleet help open up a second front for them. (In Diplomacy, you might also try to pact with them.)
Your game strategy also changes depending on your starting spot relative to the other players. If you start out sandwiched between opponents, it's gonna get hairy. If you start out sandwiched between allies then you could either develop a strong economy so you can feed your allies, fleet up so that you can double an opponent on one of the flanks (or hit across the middle), or more likely, both.
In any given game, you will almost always end up with at least one player who has devoted the time to learn the winning strategy, and that player will steamroll anyone who is not following that same strategy. The fun is in the metagame that occurs within that strategy when you have two good players facing each other (for example, in Total Annihilation, the winning strategy was more or less to spam ARM t1 tanks, and the interesting bit was who could build up the economy faster while also winning the t1 tank fight).
In the 5v5 pro games, instead of just one player who has learned the game, you often have 6 or 8 players who you might call pros. There really isn't a single "master strategy" that everyone must follow besides "Colonize your asteroid first." It's also very map-dependent.
For many people, myself included, watching replays to learn a specific winning strategy in order to play the game online without being steamrolled simply isn't fun. Not everyone has the time to devote to play the game competitively. This leads to a positive feedback loop - casual players leave the online games, making even harder for a given casual player to find a game with other casual players, resulting in more casual players leaving, etc.
Yeah, it's a shame that Sins has always had low player counts. Even after its release it only had 300 people online at once, max. A great many non-community issues drove players away (a horde of bugs and other problems, such as only 15% of the players being able to host games with games ending abruptly if the host lost his connection). I think if Sins had been released in the more polished condition it is in right now that it could gain and hold far more players.
You don't have to watch replays if you don't want to; you could learn from experience. Much of what I know came from being beaten by new strategies and tactics or from making mistakes. ("The moral of this tragic story is, send one ship in to scout first before you jump.") Sometimes you want to watch a replay to review a single event or understand how something specifically played out in a game. It is, however, a strategy game. If it weren't competitive then what fun would it be? If it weren't challenging, then what fun would it be? If it didn't offer the ability to advance in knowledge of the game and skill level, then what fun would it be? It's a world domination and combat game, not Sim City or Happy Fluffy Bunny's Garden. It's possible to lose.
The only possible solution to this problem is a functional ladder system. Unfortunately, for many online RTSs, this is not doable because the playerbase is simply too small, and the more skilled players will eventually sandbag to find a game, defeating the entire purpose.
You pretty much nailed it. One of Sins's problems is that 98% of everyone who ever bought the game never thought about trying to play it online against other people. For some reason it just didn't sell to the type of people who play games in online mutliplayer; it might have appealed more to the 4X single player crowd than the RTS multiplayer crowd. The other big problem is, as I said, a horde of problems non-game-play non-community-based problems that may have resulted in low player counts. It's too bad it wasn't released in its currently semi-polished state. It still has some problems in those areas (a player whose game crashes cannot easily rejoin the game, ruining it for the other 10 players, etc.).
Anyway, it's a great game if you are ever able to learn to play it on the pro level. All of the pros started out as noobs once, so it's possible if you stick with it. Sins does have a learning curve, but it probably wouldn't be fun nor as complex without one.