Generally, Manga gets serialized in physical compendiums of other manga titles (shonen jump/shojo beat etc). Then if a series is popular enough, they take those titles and combine the issues together with other goodies and print the volumes. The manga e-books are just the digital versions of those volumes. Webcomics start out as being released together in an individual format. I'm guessing there just isn't much of a demand to buy something most people already own in a slightly different form. I know that shonen jump now does everything digitally, but it's only been that way for a year or two and the industry as a whole still uses the old model.
