why is it that jasper and rose have the same last name hale but everyone else cullen?

Well,Rosalie wanted part of her human life to stay with her so she kept her full name,which of course is Rosalie Lillian Hale.So when Jasper and Alice came along,they decided that Rosalie and Jasper looked very similar so they passed them off as twins and Jasper took the last name 'Hale'.

Rosalie is married to Emmett but just because she's married to him doesn't mean she has to take his last name.Just like Alice doesn't have to take the last name Hale just because she's married to Jasper.