Are you sure about the predominance of non legato in Bach music? Did you read Rosalyn Tureck? And Vanda Landowska? And Lechtiwsky? Once, a great friend of mine, the harpsicordist Chiara Massini - one of the great Bach performers of our time - told me that the left hand in Bach music must simulate a cello. Do you imagine a cello playing allways in psicato ou stacatto or non legato? Could you explain better your point of view?
Rosalyn Tureck (December 14, 1913 – July 17, 2003) and Wanda Alexandra Landowska (5 July 1879 – 16 August 1959) are both too far away from Bach's time for my taste to be reliable sources. I don't want to say that on our instruments legato cannot be extremely beautiful; on the contrary. However, it was just NOT the tone ideal of Bach's time.
First of all, it is virtually impossible to achieve legato on instruments of Bach's time. Carl Philipp Emanuel Bach's ideas (8 March 1714 – 14 December 1788) about tone ideals of the time (to be found in Reginald Gerig's "Famous Pianists and Their Technique") are therefore far more credible and convincing, and he describes the tone ideal on a keyboard as "a pearl of strings". This contradicts legato (pearls on a string touch each other, but they are not "glued" to each other as in legato). He also literally condemns keyboardists who keep the keys down too long. This also clearly hints at non-legato, not at legato as we know it. Some excerpts of CPE Bach's writings can be found here:
Reginald Gerig Writes on Harpsichord TechniqueReginald Gerig Writes on J. S. Bach & the Bach SonsIt was Muzio Clementi who first started doing something similar to our present-day legato, and as far as I remember, Mozart complained about Clementi's "lack of good taste" for doing so.

EDIT: The word "legato" itself has its origin somewhere between 1805–15 (that's 50 years AFTER Bach died); < Italian, past participle of legare < Latin ligāre to bind. It is related to the Belcanto School of singing. I believe it is therefore more a "Romantic" way of musical expression, and not one of the earlier "Baroque", which emphasized form more than content.