Because BASICALLY, ballet is all about body image and is a money-making racket to boot (silly costumes and unecessary shoes).
There just AREN'T curvy ballerinas. You have to have the perfect body - thin, willowy and in perfect proportion. I well remember when my 11 year-old best mate was rejected from the Royal Ballet School because her 'shoulders were too wide'. She cried for weeks. Ballet had been her life.
Basically, they either get sick of it themselves, or they stop because they realise that their BODIES ARE IMPERFECT. Either way, the time could be better used doing some sort of more useful modern dance that you can learn in £1.99 Asda trainers which isn't dependent on having a perfect body.
Please take your daughters to something more useful instead.