Time to negotiate a raise!

I was in the same boat last employer. I wanted to be a supervisor. I tried 3 times. I come to the conclusion I was to good where I was. I left to be a Supervisor at another company. After less than 2 years I'm now the manager. The point here is keep trying. If they don't promote move on.

and this is why you should never make yourself irreplaceable.

Many people think they are ready for a promotion just because they are excelling at their current position. Maybe management thinks you are one of the people not fit for the promotion. If you really want it or need it, try working for a different company.

If they won't promote you, then it's time to advance your career at another company.

train your replacement.

Tell them that if you could do that job well, you could do the job you want to do even better

Now you know you are extremely valuable in your current position (they literally could not find someone as awesome as you), you can point to that for why they should give you a raise. If you're doing your current job so well, they should be willing to pay you more for it :) If they won't give you a raise, ask for something else like more paid vacation time or something. Side note - do not threaten to quit unless you're actually willing to quit. You'll just make everyone angry and less likely to do what you want.

Well, go and do you work a competitor's company then.

See about getting a raise then. It appears that you're valuable to them.