We are yet to find out the real reason behind leaving Project Runaway, officially by Karlie herself.
However, it seems obvious once we start connecting the dots. She has stated that she will still be a part of the show with guest appearances.
This means that things are good between her and the show, else she would not be involved at all.
And, she just gave birth to her new baby a few months ago, and Project runaway was not running then.
With her baby and responsibilities to handle the baby, she surely does not get enough time for being a part of such a show on a regular basis right now.
There is nothing else that could be a possible reason for her quitting decision, so this seems to be an accurate reason.
