I dont want to go too into detail about what happened in the comics, as the stories may find their way into the show, but one tried to kill themself, and the other was the most violent death in the comics. Also, Lori's death was the most recent main character death, but nobody cared about the dead beat neglectful mother.
I dont really consider Merle a "main character" because of how often he is absent from entire episodes, and killing Hershel seems more like a mercy killing at this point, the longer the character has to deal with being an amputee in the zombie apocalypse, the more conflict there is. Glenn, Darryl, & Michonne are probably the 3 most popular characters asides from Carl & Rick(who as far as I am conserned are unkillable), and although a large amount of the fans would be upset with Glenn, they would be even more upset with either Darryl or Michonne.