"The books don't tell us what happened to Susan. She is left alive in this world at the end, having by then turned into a rather silly, conceited young woman. But there is plenty of time for her to mend, and perhaps she will get into Aslan's country in the end--in her own way. I think whatever she had seen in Narnia she could(if she was the sort that wanted to) persuade herself, as she grew up, that it was 'all nonsense.'"

Gosh, I cried when I found out that Susan wasn't getting into Aslan's country. I refused to read the Last Battle because of it. Then I found this quote, and I was happy again, because it implied that she could get back.
As for WHY Susan forsook Narnia, I have many reasons. It must have been so hard for her to leave behind the world that she had lived in for 15 years, grown up in, and be told that she could never go back. Can you imagine having to come back to our world again after that, and know that you were nobody here, and that the beautiful country you could once call yours was shut off from you forever?
I would have hated it. I think Susan just simply could not make to adjustment and it was easier to throw herself into lipstick and Nylons than remember.
She always had the most trouble believeing in Narnia, it was always in her nature to be logical, but after she got used to it, she loved Narnia, and she cared for it. I feel so sorry for her, knowing she went through all that and then she had to lose all her family later.
I always stick to the belief that when she found out that they had all died, she remembered Narnia after it, and then she went to Aslan's country in the end. I cannot bear to leave her in this world all alone.
I always hated C.S Lewis for leaving her in this world, but he must have had his reasons, and I still think he's amazing, just Susan getting left behind and leaving Narnia without it's Gentle Queen makes me teary

Wow! What a great discussion! Many have said, and said it better than I could the reasons for Susan not being there.

But I'll add my ideas as well:

She grew up to fast. Remember, there was a war going on during the time the books take place. That could be a big factor in why Susan was already trying to act more grown up than she was.

When you are faced with a thing it is very easy to believe in it. When you are away from it, it is harder to believe in it. Such is Faith, and Susan's was week. When not in Narnia, she could no longer hold on to the believe in it, whatever the reasons. (I know I would be very sad to have been able to live in Narnia and not go back, but I'm not sure I would stop believing in it as a way to cope.)

As others have said, I hope that she would have a change of heart and one day return. Or else in some way Aslan lied by saying "Once a king or queen of Narnia, always a king or queen of Narnia." And I can' not see that Aslan would have said what he didn't mean. Of course it could just mean that once you are on the list, so to speak, you will always be remembered as a king or queen of Narnia. (Thinking about the list of Egyptian Pharaohs, and how if your name was cut out it was as if you never existed)

Though I can also see that Lewis might have wanted to show that some drift away from faith, I wonder if the Dwarfs were not a better example. "The Dwarfs are for the Dwarfs." if I remember what they say. Personally I'm hoping Susan is more like the prodigal son, and when she returns there will be much rejoicing.