Well the a is required for some of the straights which means you're less likely to hit them and if you do aa may hit a boat and also kk is at the end so can be in straights anywhere between A-9 where as 6 can be anywhere from 2-10. Plus if you set it means there is def a card that helps a hit straits.

Really, just think about all the different 5 community card combinations. Those that help kk beat aa are fewer than those that help 66 beat aa for the above reasons.