Kennedy suffered from two things: being regarded by Joss as, and resented by the W/T fans for being, the "replacement" for Tara so soon, and being lumped in with all the extra characters thrown into the show in its last season and diluting time and story many felt should have been spent on the established charatcters. (It didn't help that most of the extra characters, with the possible exception of Robin, just didn't inspire the audiene to care about them.)