Because they looked into the ark of the Lord

1 Samuel 6:1-7:1
The Philistines called for the priests and the diviners and said, “What shall we do with the ark of the Lord? Tell us how we should send it back to its place. They answered, “If you return the ark of the god of Israel, do not send it back to him without a gift; by all means send a guilt offering to him. Then you will be healed, and you will know why his hand has not been lifted from you.”
and give glory to Israel’s god. Perhaps he will lift his hand from you and your gods and your land. Why do you harden your hearts as the Egyptians and Pharaoh did? 
 If it goes up to its own territory, toward Beth Shemesh, then the Lord has brought this great disaster on us. But if it does not, then we will know that it was not his hand that struck us but that it happened to us by chance.
Then the cows went straight up toward Beth Shemesh, keeping on the road and lowing all the way; they did not turn to the right or to the left. The rulers of the Philistines followed them as far as the border of Beth Shemesh.
But God struck down some of the inhabitants of Beth Shemesh, putting seventy of them to death because they looked into the ark of the Lord. Then they sent messengers to the people of Kiriath Jearim, saying, “The Philistines have returned the ark of the Lord. Come down and take it up to your town.”
Why?
The Philistines knew they have to end the Ark of the covenant back to Israel with gifts. They listen to the word of the priests and prepare the gifts and sent it back, they check the way of the two cows goes up to its own territory, it is known that it is the Lord brought this great disaster on Philistines.
But because of Isreal in Shemesh looked into the ark of the Lord, God is holy so no one may come close to the Ark, except clean the highest preiest on a special day.
How?
God is holy so we have to be holy either, do not cheat before men and God.