p-block elements emphasis on comparative study of properties of p-block elements... including diagonal relationship and excluding methods of preparation..???

Because, methods are the ways by which we can prepare any compound. They does not categories or we can say they does not distinguish between different elements. But diagonal relationship are the matching properties of two elements located diagonally. This can distinguish between two different group of elements. And since p-block element is the study of elements in a grouped way, it emphasis more on diagonal relationship.