Our girl loved to be in plays and musicals for many years. We have a very active theater program for the youth in our area plus we have a community theatre that does a new play every other month. The youth program has almost always allowed everyone that auditioned to be in the play in some manner. Whether it was a pirate in Pirates of Penzance or a Lost Boy in Peter Pan, they didn't have lines and they did easy dance style choreography as a group. They weren't even on the stage that much.
The last one our girl was in she had a lead part and got to sing a solo. Then another person came in for a Christmas play and he only picked maybe 10 people to be in his production. All the other kids were so sad but they understood that this guy was new.
So now the kids theatre program is doing pretty much what you are talking about. The directors are all picking and choosing just a few kids to be in the plays/musicals now. The joy of auditioning and getting picked then being on stage, even for a few minutes, is gone.
Our girl has auditioned several times and hasn't been selected to be in any of the shows for almost 2 years now. She stopped auditioning after about a year.
I went to a couple of the shows and it was the same few kids in all the lead roles. Each play, same kids, different lines and costumes, but the same kids. We stopped going and spending our money on tickets.
If I had this happening in a school setting I would be sure to speak to the teacher and let them know that it's unfair for the same few kids to do everything.
I might also suggest that they instate a rule that says if you have played a lead in a production in the last year that you cannot be a lead for the rest of the school year. All programs included.
However, if a girl was in a dance and also wanted to sing in another show they wouldn't be allowed to do that. In my mind that takes away the competitive part of auditioning and working hard to get a part.
I think your child might go talk to the teacher and ask them what they can do better so they can win some parts and be able to participate.