I've been using Jetstrap and Divshot for 2 weeks on a small project. Of the two so far, I've found Divshot has more features and ease of use, yet Jetstrap is very good in its own right. I want to try Easel and I signed up, but the site seemed to be overwhelmed (by Show HN?) and it failed to load for me.
Surely we want competitors in this area, there's obviously a need for it and a number of tools competing on features and price can be good for consumers?
To answer the parent, Adobe technically does have a competitor in this space, though it's not focused on bootstrap and it's kind of complicated: http://html.adobe.com/edge/reflow/
As someone with a product in this space, I'm confident each of the current offerings listed above will diverge sufficiently in the future, specifically in their target audience. It's still so early though.