Before the wonderful days of the Wild Card Era in Major League Baseball, the American League and National League was divided into only two divisions: East and West. The New York Mets were a part of the National League East where they remain today.
Following some restructuring which then created three divisions in each league, the Mets’ rivals in the NL East fell to four. The Atlanta Braves, Miami Marlins, Philadelphia Phillies, and Washington Nationals are the three foes that they see most often and go to war with on a regular basis.
Ever since the divisions shrunk, trades between rivals have shrunk.