Hi! There are a lot problems with this code, since you didn't follow the tutorial. First, you don't see pipes because of level restarts before function addRowOfPipes called, if you disable "game.state.restart();" you'll see error in console "pipe is undefined", since you not assign pipe sprite to pipe variable.
Than, you have duplicated declaration of addOnePipe and addRowOfPipes.
Also I'll suggest you to learn how to use group that you create "this.pipes = game.add.group();", instead of adding pipes as separate sprites through custom function.
And another big problem that you are use default state and global variables (I assume you got this approach from another tutorial) instead of separate state and it properties as in the tutorial. In your code mainState, which you try to start is just empty variable.