Why did Misty leave?

I'm not talking about "she left because she had to take care of the gym." I'm asking: Why did the creators take away Misty? Yeah, I know, I'm pretty late, but I was curious about it now, so.... Was it because she wasn't Популярное in Япония (though I highly doubt it, since she's got a big fanbase there) или is it to promote the newer characters in newer games? Help me out please :P