You can mention that you have been working for so many years with excellent skills and abilities but have not been promoted at the right time. You can also mention that you are ready to face challenges but your previous job isn’t challenging. Other reasons can be that your company paid you late, or you may have traveling issues and more. Remember not to talk ill about your previous company, or if you had not achieved your task or if you were punished. Let the answer be a genuine one which can be accepted by the employer.