This is Brandon Whaley's choice, so you'll have to ask him for the definitive answer, and I expect you to do so in the comments, and he'll answer, but I think I know exactly why.
Rick Grimes is the centre of the Walking Dead title. He might not be your favourite, and he might not always have the spotlight, and he might not even be likeable sometimes, but he is the glue that holds the other disparate pieces all together. You'd call him the rock except for the fact we've seen him crumble far too many times before. But yet he remains. He's the constant, if you will, and I get the feeling he'll be the only thing we can rely on in this book that does not shy away from constantly throwing you curveballs that cannot be hit.
It doesn't help there is also a smash hit tv show based on this story. Andrew Lincoln has completely inhabited the role, even in just 3 quick episodes. He's shown the true heart of this character, the emotion, and you feel for him. If only the character knew what was to come like we do. Poor bastard.
I don't know what the others have in store for Rick, but I'm sure it's not going to be nice. Things aren't even nice for Rick, and yet he continues. You can't keep a good man down, and Rick is the best.
As is the eternal rule of The Walking Dead, no spoilers please. I'm sure some aspects will be touched upon, that's the nature of our scripts, but please no one go out and ruin one of the best kept secrets in comics.
Rick Grimes is created by Robert Kirkman and Tony Moore, with much help from Charlie Adlard over the years, and we thank them for this chance to write such a great character.