Graduated sites are listed on the footer pages of all other Stack Exchange sites. Consider this list from Stack Overflow:
(similarly on Meta.Stack Exchange )
I notice that other recently "designed" sites are up there (Biology, Chemistry)....
Based on an assumption that the shown list is based on some precalculated metric, I compared Code Review to the last on the list, ExpressionEngine® Answers, and here are the stats:
Code Review - Q&A for peer programmer code reviews
29k questions, 50k answers, 94% answered, 76k users,
47k visits/day, 34 questions/day, 4y11m site age
ExpressionEngine® Answers - ExpressionEngine® Answers Q&A for administrators, end users, developers and designers for ExpressionEngine® CMS
11k questions, 14k answers, 80% answered, 4.2k users,
1.3k visits/day, 6.0 questions/day, 3y1m site age
Sadly I can't see why this site is shown, and Code Review isn't. As such this isn't an answer as to why Code Review doesn't show, but it is a confirmation that it doesn't seem to be metric based, and that it is possibly a somewhat static list, which could need a little refactoring.