A lot of newly generated players won't move countries (or even clubs, sometimes) because they believe they are too young to do so. Pretty realistic, really. Look at real-life players like R. Lukaku who want to improve their game in a lesser-league before moving properly into the limelight.
As well as the other stuff mentioned above, it could also come down to your training/youth facilities, your manager reputation, and past experience (international footballer, for example, is more likely to tempt a player to join you).