For generations Gryffindor's and Slytherin have hated each other. Gryffindor being the smart one's and Slytherin being the evil one's. Being the evil one's, the Slytherin planned many wars amongs them, splitting up families, friendships, and even cities.
A extensive rivarily set out upon the group pulling them into the weakest times of all. The dark ages. Splitting up the Gryffindors and Slytherins is like ruining the world. Even the Ravenclaw's and Hufflepuff's can't work correctly without the balance of good and evil being in balance