Satoshi's single longest chain PoW design insures there can only be one winning perspective on the ordering and thus there is no competing ordering with ambiguously ordered double-spends. Whereas, a DAG (or any tree with multiple branches) can't unambigously define an ordering in its data structure. I know you will think we can order these in time, but I already explained upthread that there is no global clock to timestamp these nodes of the tree with. Thus the only ordering is the structure of the graph.