I play both of these games heavily though I play Starcraft even more than Sins. There are 2 reasons the majority of the Starcraft crowd won't be interested in Sins.
1. Pacing - Starcraft 2 is a much faster game, average game length is something like 15 minutes with short games in under 4 minutes and the longer games rarely stretch past 30-45 minutes. In Sins, unless it's a very small map it takes a good 15-30 minutes just to get the ball rolling and the game will drag on for 2-3 hours on a medium to large map.
2. Balance - Starcraft has an incredibly polished balance, more than any other game I've ever played. Starcraft has effective counters for everything at all stages of the game (early, middle, late). Sins was pretty well balanced in vanilla and has progressively gotten less balanced as time has gone on. Rebellion has really shredded what balance there was with so many ridiculously imbalanced things, many that are completely uncounterable. It's to the point where special rules are made by the playerbase to prevent some of the more ridiculous things to include banning factions and even specific technologies (wail of the sacrificed, etc). Titans have marginalized frigates and cruisers and aren't even balanced amongst themselves. Building a proper fleet and using proper ship counters is often made pointless by many of the changes in Rebellion. The technology trees are also horrifically imbalanced with many techs being either useless or ridiculously powerful and some factions come out either way ahead by this or way behind.
I like both of these games and I think if the dev team for Sins were to put the same kind of effort into balancing and polishing their game and Blizzard does with Starcraft I'd play Sins more than Starcraft. I really like the slower pacing and the fact that micro and hotkeys and APM with hummingbird on crystal meth timings aren't so drastically important like they are in Starcraft. But there are just so many basic flaws that creep up on a regular basis, be it balance issues, minidumps or just plain mechanics abuses (like Z axis exploits). In the end Sins has a much smaller player base with a terribly primitive online system with no ladder no matchmaker and a craptastic chat system where you can't even see who is in the same chat channel if you are in a game lobby.