Sins is bigger, more complex and interesting than Starcraft 2. That is why Starcraft 2 is more popular.

I play both. 

Thanks to Blizzard completely putting the penis in the ass of the custom and vs ai sections and concentrating almost solely on pvp and ladder, they removed a lot of the fun for me. I enjoy play against AI's.

It is not hard to go to play against a live human and meet someone who either a) loses to you and calls you names of all fashion, or beats you and is anything but sportsmanlike about it. 

As it is, I haven't really played SC2 in weeks. I played a few games.. but eh. It might have lost its replay value. Starcraft 1 in a lot of ways, was more complex than Starcraft 2.