Keep in mind that most of the students who are placed in honors classes are those who easily understand the concepts being taught and can keep up with the pace. If your daughter sometimes does not understand the math in her current class and works more slowly than most other students in her class, then the teachers might feel that the honors class may be too difficult for her.
Talk with the teachers about the possibility of letting your daughter try the more difficult honors class for a short time to see if she can handle it. If they feel that the change would not be in her best interest, accept that and help your daughter see the strengths that she has in other areas.