Because having some riders coming down whilst others are going up is a recipe for disaster. Also, the climb today is 33km long. Val Thorens will have paid in the order of €1m to host a stage finish, the hotels and everything will be booked for riders and race staff. If i'm dealing in pie in the sky, you could head north over the Grand Bernard and up to Annecy but we've no idea what those roads are like adn you can't organise a stage of the Tour in an afternoon.
This is only the second time in Tour history a stage has been shortened for adverse weather, I really don't see what they could do other than this.