if the zombies can be evaded as easily as Morgan has been evading them ["Stay quiet at night and they wander away in the morning"] then the zombies really should not have been triumphant.
Well, it was heavily implied that it started out as an epidemic. People were getting sick with a fever, and it was a disease they didn't know how to fight, so a lot of people ended up getting infected. It wasn't until they died from the fever that they became zombies.
It's not like somebody became a zombie and started spreading their zombiness around.