iCarly was more successful than other kidcoms. iCarly's creator (Dan Schneider), said on his blog that those type of shows typically last only 3-4 seasons.
Starting to think that's the case as kids usually grow up a LOT in those years. Sure, there are 20-30 year olds that can play teens forever, and kids who are cursed with that thing that Gary Coleman/The Speedy Alkaselzter Guy/Walter Tetley had... but for the most part, the precocious kids go through puberty and become something else. That's basically the shelf life of their age group. It's like how Arthur fans constantly complain about new voice actors that
have to come in when the kids' voices change.