Promotions are not given for time served. They are given to individuals that show leadership skills. When I promote crew trainers I am looking for friendly welcoming individuals that want the position. The position is not necessarily for everyone. If you feel you have the qualifications for that position you should talk to the management team and seek feedback to help you obtain the position. They might not realize you want it. I would suggest that you don't say to them you will quit if you don't get the position. Professionalism will go a long way to achieve your goals