There could be lots of reasons, OP.
As others have said, the behaviour of the child could be enough to warrant it eg they harm others in some way. (Though you've said this is not the case).
The mother could simply be an awful person.
Or the mother could be human, but weak/unable to cope. For various reason - MH issues on either side, co-ercement by an abusive partner, or poor (but not violent or criminal) behaviour from the child which the parent just can't cope with any more.
It must be awful for you OP, whatever the reason. 
As a pp kind of said, I think even as adults we hold our parents (particularly mothers) to high standards, and expect them to always be able to be there for us no matter what. We judge them more harshly thab we judge ourselves.
But the reality is that mothers are human. Even the best are weak, flawed, sometimes selfish.
And some, perhaps many are just not good people.
Though it's particularly shit when it's their children who have to pick up the pieces left behind by that reality.