Although many people see cos-players as people who worship anime to it's very core, it isn't just those people who have spent a major portion of their lives watching anime or reading manga. A majority of those who cos-play imitate characters from their favorite movies, TV shows, and video games as well, not just anime. In large part, people (like myself) like to pretend to be that person in very sparing intervals of their lives, most often being from something that they have created themselves. (i.e: Original storyline, Fan-Fictions, or fan-spawned fictions from their favorite storyline)
There is no doubt that this trend originated from the land of the rising sun, especially when people see the Tokyo Game Show on G4, but my opinion on why there is an explosion of popularity in this trend is.... well, they don't just want to be themselves, but rather an expansion of themselves in an alternate reality, which isn't really all that bad. Escape is very much needed in today's society, and in all actuality, all of us need to join into this trend at one time in our lives, and that's what makes this trend so popular: the need to escape from ourselves and be something else. In brief terms, cos-play is so popular because people have a need to get away from themselves and have a little fun before going back to real life.