It might just be that there are so many more SJ's to choose from. I know that no type statistics are definite, but everything I've ever read points to S's being in the majority over N's.
In some places I've read the percentage of SP's and SJ's is about the same. In others I've read SJ's largely outnumber SP's. In either case, you're certainly more likely to run into more SJ's than either of the two N temperaments.
In addition, like nevermore said, S's tend to understand each other on a lot of basic levels moreso than they understand N's. So with that as a starting point, it's back to the typical J vs. P argument....the one that J's need P's to help them loosen up and P's need J's to help them settle down and organize. P's find needed stability in J's and J's find thrill and excitement in P's. This is even more pronounced in SJ's and SP's than NJ's and NP's.