2 main reasons.
1. Unique races. Not just slightly different, but almost totally unique. No other RTS game comes even close to offering 3 near 100% unique races. Each race has little in common past collecting resources. This is very underrated when it comes to SC2's popularity.
2. Balance. With these unique races comes a great challenge in keeping the game balanced. If the game was unbalanced you'd see one race and only one race played at the highest levels.
At least one of these two things are largely missing from virtually all other RTS games.
Also, outside of the standard competitive multiplayer aspect, you have the immensely popular custom maps made from the map editor. No other game has the custom map making community that starcraft has (other than WC3 of course).