The main reason should be their biographies being put together in the same chapter by Sima Qian 司馬遷 in his Shiji 史記, or Records of the Grand Historian . Nonetheless, earlier occurences before Shiju can still be found, i.e. from Xunzi 荀子, Han Feizi 韓非子, and Lüshi Chunqiu 呂氏春秋. The mindset behind this combination should that they two are regarded as the two best strategists before Han.
The state was called only Wu by contemporary people. Sun-Wu is a term used by later scholars to differentiate it with many other states on history sharing that name. In fact, all cases of Sun-Wu in Sanguozhi 三國志 are referring to the duo instead of the state.