20/8.5 is on the loose side, and pretty passive (you're not even raising 50% of your hands that you play).

6max there is MUCH more action than full ring.
a) blinds come around faster. this means you're forced to play more hands, simply because if you don't you get blinded to death and you lose.
b) since blinds come around faster, the games are more aggressive.
c) since the games are more aggressive, there's more bluffing
d) since there's more bluffing, that's where your AA/KK gets paid off huge