The Philistine rulers consider the tumours to be a result of chance and not from the Lord (cf. 1 Samuel 6:9).1 To their minds the Lord has been defeated in battle, and thus he does not have the power to influence events in Philistia where Dagon rules. If the ark is sent to Gath, and that city remains unaffected by tumours, that would serve to confirm that the tumours were as a result of chance and not from the Lord.2 Gath was a city further away from the coast. Since the plagues were associated with rats, and they were found mainly on the coast, then sending the ark to Gath (where there were fewer rats) would make sense from a human point of view.3
8 So they sent and gathered together all the lords of the Philistines and said, “What shall we do with the ark of the God of Israel?” They answered, “Let the ark of the God of Israel be brought around to Gath.” So they brought the ark of the God of Israel there.