I might be entirely off-base...
Because your question isn't entirely clear. You might be asking why you can't use a star tracker to track asteroids in an asteroid belt. I wonder this because if your discussion is literally asking why one couldn't use a star tracker to track stars in an asteroid belt, then it only makes sense if you're thinking asteroid belts are like what are shown in Star Wars, which in our solar system isn't true. (It might not be true in any solar system. That many asteroids would be pulled together to form planets....)
From this question over at Worldbuilding we learn that our own asteroid belt is so empty that the New Horizons space probe sailed right through it and nobody shed a drop of sweat over it. I believe I once read that if all the mass in our asteroid belt were brought together it wouldn't add up to the mass of our moon.
But if you're asking about tracking asteroids...
You're absolutely correct — it won't work. Star Tracker tracks emitted light. Bright emitted light. Asteroids aren't known for emitting bright light or even reflecting bright light, so the tool's value for tracking them is limited. Another issue you probably didn't think of (and, frankly, I don't know enough about) is whether or not the asteroids are moving too fast for a tracking system that's designed to track what, for all intent and purposes, are stationary objects.
But what really becomes a hard no is how difficult it would be to detect your location based solely on asteroids. You'd need to know exactly which asteroids you detected and where they were expected to be at the moment you performed the analysis. Yuck. Remember that such a tracker would have trouble due to the speed of the asteroids? Even if you could track them... there's still the effort of extracting useful information from the data. There are a lot of asteroids in our belt the size of tennis balls and smaller.
What other sensors could be used?
You're stuck with just one at our current level of technology: radar. So if we assume (via magic) that our radar is infinitely precise (could track a tennis ball light years away), then we must ask what's the point of tracking an asteroid that's a light-hour away? The value of such a radar system is based on how quickly your ship can react to the detection of an imminent threat. And that's a function of your velocity, the asteroid's velocity, and how far away you can detect the asteroid. The odds are in my favor that an asteroid a light-hour away will take a year and a half to hit you.
But let's ignore all that. The value of radar is directly related to the power of the pulse and the time between pulses. Check out this quick tutorial about calculating maximum useful radar range. You can easily detect imminent threats — so long as you carry a big enough array and a big enough power plant. This, because you really don't need to care about objects a year and a half away.
But we're back to your question — could you use radar to track your location in an asteroid belt? Theoretically it can be done, if you have an accurate enough radar pushing far enough out into space with an accurate enough map of all the flotsam and jetsam in the belt. But I believe it stretches credulity.
Unless I misunderstood your question. If I did, let me know and I'll delete this answer.