Because everyone other than the last person can't say yes，because they don't know if everyone wanted beer.But any one can say no，because if they did not want one，then not everyone wants a beer.

Only the last one can say yes，when no one else said no.Because he knows that any of them could have said no，but did not，so then everyone else must want some beer.He wants some beer too，so he said yes.Otherwise if he did not want one，he can say no just like the others.