Question on AlwaysON and pre-reqs

Hi All,

A question on AlwaysON. I want to know why did AlwaysON needs a windows cluster as a pre-req, then install standalone sql server instances and then configure AlwaysON AG's on top of it. Why can't we just use same old database mirroring i.e. just take 4 standalone sql server's and configure AlwaysON just as we do with Database Mirroring? I still don't get why why do we need a windows Cluster as per-requiste? Why did Microsoft felt or did they made use of any windows Clustering feature and combined with database mirroring? Any anyone can explain why?

Thank you and appreciate your help.