One could assume, since he continued writing in the idiom he had mastered, that he wasn't too interested in the newly developing classical style.
That could be it.Although one cant say that Bach wasn't experimental with the music he created. Like with the Chromatic Fantasy and Fugue in D Minor or (little geek fact) if you look in bar 20 of the e major prelude from the well tempered clavier book 1 he used a Gmaj7#5 chord which is out of the melodic minor scale and which wasnt again used in music until the 20th century So what I like to think is that maybe the early classical music just wasn't his cup of tea.Who knows maybe he would have been into atonal music tho (although I doubt it)