IME, in independent schools, it is typical for the school administration to limit class sizes for the most advanced classes and to determine which students take those classes. I have heard that the reasoning is that student motivation and consistency plays a large role. Parents aren’t in a position to evaluate those things in school, so the school determines placement based on space available and relative performance.
You can always ask your child’s division head why she isn’t in the honors track or what she can do to join the honors track in the future.
If the regular math track includes AP Calc AB in senior year, then that’s a college prep track, too.