Why would they do this? Hershel's farm was the setting for season 2, a whopping three issues from the comic. Then they decide to cast aside the story up until the governor and say the hell with the 15 issues of the book in between?
I'm all about going on a different path, but at the same time it seems like they are rushing the story because they think they will lose viewers or something if the show becomes too slow. There's no reason they couldn't stay just a "little" more on track with the book.
Here's to hoping season three isn't a let down. I think I'm still a little disgruntled that they took a pun at Tyreese's character to include someone names "T-dog" who has a secondary role in the show.
In the comics, most of the prison story until they met the governor involved characters that either aren't in the show (Tyrese and his daughter and her boyfriend) or haven't been introduced yet (The prisoners holed up in the cafeteria). I'm not even sure they will introduce Axl and the other inmates. If they don't, once the group gets inside the prison, there isn't much story from the original source material.