Some people can, some people can't. Neither position is wrong, it's just a matter of feelings and personalities. It depends on how the relationship ends, if one person feels more than the other etc. It may be that you took whatever happened more seriously than she did.
Personally I'm not friends with any of my ex's not because it would be upsetting but simply because that part of my life is done with. If we worked together, or were at school together then that would be a different situation. Perhaps she just doesn't want to lose you as a friend? The only way of finding out is by asking her.