Here's my opinion...
Sins is NOT meant to be a singleplayer game...really it isn't...I'm not saying people shouldn't play SP, what I'm saying is that the strengths of Sins make it better for MP than for SP....
Compared to most SP games, Sins has very little depth (hence why many refer to it as a light 4x game)...consider the Total War games with their epic campaigns and subtle intricacies like spying, assination, rebellions, troop morale/fatigue, family characters, etc...or Civilization with its multitude of playable factions and finer points like civics, religion, espionage, promotions, happiness, great wonders, etc...or Galactic Civilizations, which has a very nice, thoughtful, and detailed lore/campaign in addition to tons of "4X like" game mechanics such as random events, faction custimization, ship customization, planetary micromanagement, etc...
Many of the things I mentioned above fall into a broader category of "making choices"...in Sins, you basically pick one out of 6 factions and that's about it...one might argue the cap/titan/SB upgrades or abilities you pick are "choices", but that is trivial to what other SP games offer...
Other than great aesthetics (both visual and audio), Sins just doesn't have all that much to it in terms of the SP experience...all you really have is a streamlined tech tree and counter system based off of pre-designed ships...but that is exactly what makes for an excellent MP RTS game...
Sins has a limited number elements in terms of number of ship types and structures, but the complex relationships between all those entities make for a very unique and interesting counter system or method of developing an empire (hence the "easy to learn, hard to master")...in principle, Sins uses the "less is more" to create a variety of strategies a player can use to win...
Sins has a streamlined tech tree and empire management (no microing culture expansion or trade ships, for example)...it has a great user interface...it has a strategically complex and mechanically simple combat system...basically, Sins has all the core elements needed to make for an absolutely fantastic MP game...
Unfortunately, the devs have the exact opposite philosophy...despite having an MP jewel that lacks in SP qualities, they prefer to focus on their SP experience at the expense of improved MP...basically, you have an incoherence...it's like trying to make a sprinter become a marathon runner and a marathon runner become a sprinter...
The SP people lose interest after awhile because the game isn't your typical in-depth 4X game...the MP people lose interest because of balance issues, desyncs, and MDs (which still happen btw)...that you have to wait half an hour just to get a game going drives off people even more, making it a self-perpetuating issue...
There was a window, a time when Sins maybe could have become truly big...but that window is long and gone...it's not the devs fault as much as it is their choice...they made the game the way they wanted it to be made....it's a fantastic game, but I think what the devs saw for their game (great SP experience) and what actually happened (massive MP potential) just didn't mesh well...it is what it is...