The Shulhan ‘arukh would not have had its staying power as a work of enormous cultural authority had Rabbi Isserles’ glosses with it
…it is safe to say that the Shulhan ‘arukh would not have had its staying power as a work of enormous cultural authority had it not become an entirely different text when it appeared in Kraków in 1578–1580 with the glosses of Moses Isserles. Isserles, one of the towering figures of early modern Polish Jewish life, had been at work on his own law code for some time when he learned of Karo’s project. Rather than compete, he decided to append his own glosses with what he claimed were the Ashkenazic customs and practices. In this edition of the Shulhan ‘arukh, one finds a central dynamic of early modern Jewish history on the pages of a printed book: the coexistence, competition, and tension between Ashkenazim and Sephardim. Indeed, the very categories of Ashkenazic and Sephardic are thrown into relief by the reactions to Isserles’ glosses. Thus Hayim ben Bezalel, brother of the famed Maharal, had little patience for Isserles’ attempt to summarize all of Ashkenazic tradition in his glosses and took it as a form of cultural imperialism and an erasure of difference among Ashkenazic and Polish practices from different regions.
Yaacob Dweck, “What Is a Jewish Book?,” AJS Review 34, No. 2 (November 2010), 370.