Board Thread:Theories and Speculation/@comment-30168130-20161223235115/@comment-4912646-20161224030029

Using kekkei genkai and kekkei tōta requires more than just knowing how to use the composite natures, you also need specific genetic traits in order to combine them. In the manga itself we see characters like Kakashi, Kurotsuchi, and Mei who can use three or more natures but don't have a kekkei tōta (and Kakashi doesn't even have a nature kekkei genkai). And as revealed in the latest databook, a large number of characters can use more than two natures, yet Ōnoki and Mū are the only known individuals who can use a kekkei tōta.