Why are pigs good 'mixing vessels' for new influenza strains/avian influenza?

Pigs have both 2.3 and 2.6 sialic acids on their receptors in the respiratory tract. humans only have 2.6 sialic acid on their receptors in the upper respiratory tract. avian influenza prefers to bind to 2.3 sialic acids, which it can do in pigs. there it can mutate and also bind to 2.6 sialic acids. now it can also bind to 2.6 sialic acids in humans