I think it has to do with jQuery becoming the dominant standard for everything web, that said it's not too much code so removing both the dependency on jQuery and the dependency on underscore should be fairly simple.
The trend of jQuery becoming the dominant standard for everything web has been reversing for 2-3 years now, and that is only going to accelerate as more people are able to abandon support for IE6, IE7 and IE8. JavaScript in most modern browsers makes most of jQuery redundant.
A dependency on underscore strikes me as usually being more reasonable if you use more than 1-2 functions from the library. I regularly use many functions from underscore when I include it in a project since it includes many utility functions for functional programming that are missing from the basic prototypes in JavaScript.
