When they last expanded, they needed to have an even amount of teams in both leagues, or they would always be playing an interleague series for the entire year. MLB recognized this and when Tampa Bay and Arizona joined in 1998, they switched the Milwaukee Brewers (Bud Selig's former team) to the NL, giving the NL 16, and the AL 14. MLB has always wanted to keep interleague games at certain times of the year, and all played together, so they went ahead with the way they have things now. Interleague play is considered a "special attraction" and having one interleague series always being played would've made it lose some of its luster.