Bach was a master of transcription and recycling, as violinist especially should know(he used the prelude to the third partita in a cantana for the 1st vlin part). Cello suites could have very well been in a version for viola. Like Mozart, Bach prefered to be in the middle of the harmony, therefore his preference for viola.
(Quoted from wikipedia)
"Her interpretation of these suites made such an impression on Pablo Casals that he is alleged to have admitted to her, immediately after she played for him, that they sounded better on the viola than on the cello."
This may be isolated to one violist though. Bach's music like the majority of Baroque music is universal. For example, in a viola class, I once played a transcription of Invention 1 for two violas in F(by my viola teacher). I once heard an arrangement of it on from the top for sax and Double bass.
I forgot where I saw it but Bach's wife said you weren't supposed to bother bach if he locked himself in his room and played the harpsichord or viola.
