You explain in your text, exactly why you haved not been promoted to the post of manager. I will try to explain you by my opinions and my visions of McDo why you're not a manager actually.
Like you said in your letter, you're probably the best worker of the restaurant, by making closing very faster and by working during the afternoon before closing. You're probably the best employes because you haved made shift or 18 hours in a day. They don't want to lost you're kind of worker by promote you to the post of manager.
Have you ever taugh that if you become manager, you will do you're eight hours a days, 5 days a week to principally SUPERVISE and HELPING the others crew members. You'll not be abble to make 18 hours a day because a manager can't do it. You are part of the categories of the perfect employes, the employes that every boss are dreaming to and it's why it's better for him to let you in this categories because you're always there to save his life when any other crews can't do what YOU DO!!!
It's a part of the probs with MCDO. If you do all they want and more, you'll have your month's employees plate, the felicitation of you're boss but you'll not be promote to the post of manager because you're a too perfect crew member. You have a chance to be promote, only if you want to leave this job because you're not a manager. It's the problems we have when we're the best ones!!!
Do what you think is best for you and don't be shy to be involve!!!
I continue to beleive that the unions are the best things that McDonald's employes can have because it will putt some discipline from our employers!!!