Nostalgia.
It is also a family ride that really everyone can ride. No worries about being too short, pregnant, or having other health issues.
It is not scary.
It is located in Fantasyland.
It has always been popular, but I don't remember it ever being as bad as it is now, especially right at rope drop.
My thought is that everyone rushes back to ride 7DMT, and when they see the long line already formed at 9:00 am, they look for something else to do instead. And what ride just happens to be directly across from 7DMT? Peter Pan!
This is my reasoning behind why the line for PP is getting long so early in the morning. 7DMT is funneling rope drop people right to it.
You add to this now being limited to only 3 FP which many people use on the mountains and the line has become longer than ever.