Many laws exist in the Talmud that don't exist in Shulchan Aruch.
The most common cases are those that don't apply nowadays, like sacrifices etc... There are other areas that I don't recall being in Shulchan Aruch like Loshon Hora.
My best guess is that the Shulchan Aruch is built around the Tur and so you'd have to go and see how the Tur decided on which areas of Halacha to include.