Actually SZS sold well enough that it was consistently ranked in the NY Times Top Ten Manga List.
The NY Times Manga List is not very accurate and even NYT admits it. It only covers the sales through the distributors, which aren't really sales and more of just units shipped. SZS volumes could be getting some really high returns. This is what killed Tokyopop.
Top it all off, quite a few online stores do their own distributing and they don't supply their numbers to NYT.