I have theories on why he was left out, which could or could not be actuality, some of these theories may even coincide.
1) Nintendo could be saving some of the largely requested characters for DLC as a guaranteed sale.
2) Nintendo has a personal bias against including King K. Rool because he wasn't created by Nintendo. Nintendo may carry some pride towards their characters and may feel given the Rare fiasco may have some bad blood towards Rare created characters.
3) Sakurai believed K. Rool wouldn't offer anything unique to the roster in regards to play style and would just be an additional heavy character.
1) Nintendo could be saving some of the largely requested characters for DLC as a guaranteed sale.
2) Nintendo has a personal bias against including King K. Rool because he wasn't created by Nintendo. Nintendo may carry some pride towards their characters and may feel given the Rare fiasco may have some bad blood towards Rare created characters.
3) Sakurai believed K. Rool wouldn't offer anything unique to the roster in regards to play style and would just be an additional heavy character.