Speaking only of the various printings of the Kitzur Shulchan Aruch, the reason why it is usually printed with footnotes is because those footnotes are intended to tell us about other poskim we usually hold like (in preference to the Kitzur Shulchan Aruch).
The Kitzur Shulchan Aruch is so prevalant because it's a convenient summary of a lot of halacha (fits in one volume, if you buy an English translation, you don't need to buy a new bookshelf to hold it, it covers important basics in Yoreh Deah, Even HaEzer, and Choshen Mishpat, and it doesn't have the complicated format that a Mishnah Berurah does). However, since the general public holds like the Mishnah Berurah instead, the footnotes are there to tell people how the Mishnah Berurah holds. Similarly, the new ArtScroll translation covers the opinions of modern poskim like R' Moshe Feinstein (who the general American public holds like). Similarly, in Israeli printings of the Kitzur Shulchan Aruch, you can find an appendix written by the Chazon Ish about halachot that apply only to Eretz Yisrael.
(I do think that R' Mordechai Eliyahu's version of the Kitzur with footnotes from Ben Ish Chai has a different purpose. Since Sepharadim are very easily able to get their hands on a copy of the Ben Ish Chai, and it's not too much larger in Hebrew or English than the Kitzur is, there doesn't really seem to be a need to print the Kitzur Shulchan Aruch simply to make a comprehensive modern sefer of halacha avaialable. In this case, I think the point really is to be able to learn the psak of different poskim/eidot comparatively.)