Because J&T were designated to assist N&S, N&S thought they were superior to them and they didn't need to inform J&T of their decision. All that J&T could do was to follow their orders.
What was the principal cause of their awkward situation?
Americans tend to bond their activities with exact contracts. When J&T promised to arrive in five minutes, N&S thought they made a contract. And when J&T failed to arrive as they had promised, N&S regarded it as the breach of the contract, so they had the right to go away without informing.
N&S didn't considered traffic jam to be the reasonable excuse for lateness. They thought it was J&T's own fault that resulted in their lateness.
It was the different time value that led to the awkward situation. When J&T promised to arrive in "five minutes", they just indicated that they would try their best to arrive soon, more or less than five minutes. However, N&S the "five minutes" was precisely 300 seconds. And they thought that J&T's lateness would also break down their following activities.
