HardyGirl: "The Adventure Team came after the military figures, Wrathdemon. Even though they kept the name GIJoe, the concept was completely different. To that end, a new look was presented, something cooler, and more 70s."
I think it had something to do with the Vietnam war. Parents didn't like the idea of buying military toys for their kids at that time, so GI Joe was reworked into an adventurer. I'm pretty sure that was the story I read somewhere. Whatever the reason, I love the Adventure Team, although I won't be marrying it anytime soon.