They'd lost. Plain and simple. So in a desperate (albeit cliche') bid to protect their future, they pooled the last of their power to imbue one man with the power to save the world, although it isn't who you think...

It's been said that Harry's horcrux/scar could have negatively affected the Dursley family, causing them to mistreat Harry. What if it was wizard-kind as a whole that affected them, however? One tiny difference in a conversation that almost never happened will change the fate of the wizarding world as a whole. The real question is, is the change for the better, or worse?