The community was fragmented by microexpansions. By the time of the last one, the community was split between four different versions of the game. New players coming in to test out multiplayer could find no one to play a game with: they left, and saw no reason to buy the microexpansions for online play since no one was playing online.
Frequent "desyncs" lead to issues where a game would have to be completely abandoned and restarted. The lack of a solid tutorial (the one they had was not adequate to explain the intricacies of the game) created players unprepared for online play, ensuring they would be defeated and often in annoying ways (scout rush etc) and that drove away newbies.
Ironically, a lack of a single player campaign also may have contributed. Without a solid, "happy" game experience to endear them to the title, for many people Sins was something they played a couple games of and, not feeling there was really an objective, these people left the game aside.
The choice to "require" Impulse to patch the game, while perhaps a sound judgement to deal with piracy, lead to refuseniks who did not update (and therefore did not play the game online.)
The length of the games meant those with limited time (two hours or less) could not find a game they could complete most of the time. These players also had problems with the online community.
The modding support was average, but without being able to download new modded maps directly in the client, new and innovative game mods did not "catch on" and suffered from a dearth of players. It encouraged fragmentation between those who played popular mods and those who didn't, further dividing the community.
Somewhere I have a really detailed breakdown of things involved in Sins' eventual decline of the online community. Not all of these factors were apparent on launch, and some of these factors counted more than others. Any given one could only have cost the online community a couple of "regulars," but keep in mind they do add up, and Sins already had a small community that could not afford to lose any more playerbase.
Are these things Stardocks' fault? Not necessarily. They published the title, they did not develop it. I am not criticizing anyone, just giving my views on what seemed to harm the game's playerbase.