Q. Why do Sephardim and Ashkenazim have different laws?
A. The two groups have the same halachah, though there are variations in their customs, styles and interpretations. Each group has its own liturgical traditions and Hebrew pronunciation. When there is a difference between the view of Joseph Karo, compiler of the Shulchan Aruch, and Moses Isserles, the compiler of the glosses to Karo’s code, the Sephardim tend to follow Karo and the Ashkenazim follow Isserles.
This especially applies to the food laws. Because Karo insists on glatt kosher meat (where the animal’s lung is smooth without adhesions), the Sephardim follow suit, though many Ashkenazim have adopted the practice. Karo allows glass vessels for both milk and meat provided they are rinsed in between, whilst Ashkenazim are stricter. Following a ruling by Karo, Sephardim eat rice on Pesach whilst Ashkenazim do not – not that rice is forbidden in itself, but Sephardi authorities draw different conclusions from the Talmudic debate (Pes. 35a). In some communities, Ashkenazim eat fish, not meat, on Friday night, a custom mentioned in the Talmud (Shab. 118b).
The best work in English on the whole subject is “Ashkenazim and Sephardim” by Dr HJ Zimmels, former principal of Jews’ College, London, published by Oxford University Press.