From what I've read, eligibility is the 10 smallest-market teams and the 10 lowest-revenue teams. Several teams are in both lists and are called crossover teams. Because of that, the final list is 13 teams.
These teams aren't eligible:
AL East - Yankees, Red Sox, Toronto,
NL East- Philly, Atlanta, Mets, Nationals
AL Central - Detroit, White Sox, Twins
NL Central - Cubs, Astros
AL West - Texas, Angels, Mariners
NL West - Giants, Dodgers,