This is why because, you are only previewing your newsletter, technically there is no other data associated with it. Hence only the placeholders get displayed, the custom fields and the unsubscribe link will show up when you send your newsletter to your contact list.
Want to have a trail run for your campaign? Do you want to test the quality of the service we offer? One of our customer advisers will call you to discuss about your requirement and set up a free trail account for you.