People just get bored of it. It seems fun... "I love nappies, wearing all the time must be great!" But then it gets to be a bit of a slog, it stops feeling special. Then you realise that one of the best things about nappies is the anticipation and excitement that makes you look forward to one... It becomes less a fun thing to do and more of a job, a hassle or inconvenience. Perhaps a small one, but still one that you don't need to give yourself.
I see a lot of people say the same thing. After a week, two weeks, a month, who knows... People start to get bored of it or no longer find it as fun as they used to.