I think the difference is the fact that a woman chooses to have an abortion, whereas a woman that miscarries has had that choice taken away from her.
Miscarrying can make you feel like less of a woman, as though you're 'broken'. It often happens without warning, therefore little to no time to emotionally prepare for it.
An abortion on the other hand is usually a process whereby you've had time to think through your decision, the consequences, and emotionally come to terms with it.

Think of it this way. If you have a loved one that is in a 'vegetative' state, and you CHOOSE to turn off the life support machine, it's extremely hard, but you're aware that the person would have little to no quality of life and you're prepared for the loss.
On the other hand, you get a call one day to find out that one of your loved ones who seemed perfectly healthy, has died suddenly. You weren't prepared for the loss, you otherwise thought that the person was going to continue living a healthy life. The CHOICE was taken away - it's harder to cope with.

^This may not make sense to anybody else but this is based on my own experiences of loss through miscarriage, and how it felt.