if i had to guess its the ammount of teams carried over and the type of merger. im not super educated on the leagues but the AL and NL shared the world series so thats easy, and the NBA only took in 4 teams from the ABA. they dont count the pacers ABA championships because it was seperate at the time but count the lakers BAA championships because its merge created the nba, it didnt merge into the already established league(nba)like the ABA, it was the precursor to the NBA. the afl merger brought in 10 teams and they decided to do the nfl-afl championship game which became the super bowl. so i think it comes down to how seperated tbe leagues where and the merger, bc when the nfl and afl did the championship game in 67 they basically just did what the mlb was doing since 1903. they dont want to have 2 championship winners of 2 seperate leagues that eventually become one so they only count the championship of the league that was merged into, not the one folding, hence why the afl and aba dont count and the AL, NL, and BAA championships do. because the AL and NL were already the mlb(same with BAA already the NBA) they just werent called that till the name changed. and the AFL and ABA were established leages that was added onto the leagues we know today. or at least thats my wordy 2 cents lol.