As you see. What can be the reason? I added it to communities list about a week or two ago.
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Area51 is... special. It is being run on a fork of the Stack Exchange 2.0 platform, but the codebase is so far removed from the current 2.0 codebase used by the rest of the Stack Exchange network that most changes never make it across.
As such, it just doesn't support exporting your reputation for use in that dropdown box.
Once upon a time a new version of Area51 was in the works but the last anyone has heard about that idea was 2 years ago now.