I really enjoy inking and the look of an artist's style with raw, unfiltered lineart can be wickedly appealing. Some of my favourite webcomics are achromatic or with a very limited colour scale and it's truly such a nice feeling on the eyes to look through. However...
... digital media presentation on electronic displays challenges the way our eyes see light and colour. I think it's a platform that calls to experimentation and exploration with colours otherwise impossible to replicate on paper. If you're a creator who's dipped into physical print publishing, you've probably seen printers' guidelines about CMYK vs. RGB and how our digital devices' backlit screens give a crisp, vivid (albeit false) interpretation of certain hues. I feel like that impressive colour presentation really influences an artist's decision to produce a webcomic in full colour. It's dramatic. It's fluid. It's visually exciting.
Ultimately, it's about grabbing the reader -- who's probably already on the go, distracted by competitive ads, congested commute, multiple conversations, urgent errands, and a million other sensory loads -- with something new, that they haven't seen yet (or at least very often). A bright, energetic colour palette interpreted through a digitally sourced light can help a lot.