I don't think Sins has all that big of an audience to begin with...
According to wikipedia (the most trustworthy source in the universe), as of 2008 Sins sold 500,000 units....
However, only 100,000 of those were digital downloads...so, it is quite possible that many people who bought sins aren't even aware of ICO, Impulse, updates, or the expansions...
On a good day Sins Diplomacy may have over 100 people...that is .02% of total owners...
Let's compare this to Halo 3, which has sold over 8 million copies (by Jan. 2008, anyway)...it was not uncommon for over 1 million unique users to play online at any given day during Halo 3's peak...I'm not a statistician but if people averaged 4 hours a day (which is probably an overestimate) then at any given time you could expect around 160,000 people online, which is only 2% of the total amount of owners...
That is 100 times more people proportionally than sins...
Why might things be like this? Maybe because ICO sucks, but I seriously doubt that is the only (or even biggest) reason....
One, Sins is an entirely different audience, and many people who bought sins are inherently inclined to single play...
Two, as stated earlier many people of sins (I'd easily believe over half) aren't even aware of online MP...
Third, Sins has mods that some people love...I for one don't play on ICO because I prefer a mod and can play LAN games with friends...don't have numbers here, but I know I'm not the only one (of the 6 friends I know that own sins, none of them have played on ICO)...
So, both absolute and ratio wise, sins MP audience is significantly smaller than most games...in addition, Ironclad and SD are very small companies with very small budgets...
It really does not surprise me they don't make a better multiplayer...there are simply so few people who play to begin with, and a better ICO is likely only going to attract people who already own the game and know about online MP to begin with...there's simply no money to be made...
Sins is not new, has balance issues, and doesn't have a large staff to oversee it...even with a pristine MP, I highly doubt sales are going to go up enough to justify it...if I was Stardock, I'd be more interested in making damn sure my next title is good than piddling around with an old game that's already tapped its market...that they still release updates and are working on a 3rd expansion is far more than we should have ever expected...