I am confused that the Muwatta was not included in the six books of hadith, considering that it was made earlier than all of the six books, so it should be even more reliable then the six books.

Might it be because all of the sahih hadith in Muwatta is already present in the six books?

I am confused that the Muwatta was not included in the six books of hadith, considering that it was made earlier than all of the six books, so it should be even more reliable then the six books.