Though it is possible that a new cart would be harder to pull and manoeuvre than an older one, it is most likely required for the purposes of ritual purity. A new cart is ritually clean.1 Being unaware of the Mosaic law, the Philistines would not have known that the ark was meant to be carried by Levites on poles (Exodus 25:13–14). When it comes to the cows, they were a final test to confirm beyond a doubt if the tumours had been from the Lord or somewhere else (see 1 Samuel 6:9). The cows pulling the cart—being untrained and having recently had their calves removed from them—would be much more inclined to go back to their suckling calves than to walk up the road.2 If they walked up the road it could only be because of a supernatural intervention by the Lord since they would have been acting against their nature.3
7 Now then, take and prepare a new cart and two milk cows on which there has never come a yoke, and yoke the cows to the cart, but take their calves home, away from them.