The preview is for people editing it to see what it will look like they should know what the tokens are for. There is not going to be token replacement in the preview because it is not sent to a subscriber, it is sent to an arbitrary email entered by the user, and expected to be the email of the person who is editing it.
I'd like to add more token replacement at some point but there are problems to be solved such as the fact that people can subscribe without registering on the site by just entering their email address and nothing more. Probably never going to add token replacement for custom profile fields even if we do manage to solve the problem with adding a few more token replacements, it will be just core properties like FirstName and LastName (which have fields in the mp_Users table so they are core fields even though they are not enabled by default in mojoProfile.config). It is going to be a difficult problem to solve because people have existing subscribers without anything to replace additional tokens with.
Best,
Joe