Issi noho I understand what you are trying to say I really do BUT…
You are making huge assumptions. The Captain is in command of the aircraft but he doesn't have the information to make a decision to divert.
The note was received in flight. Who wrote it? Are they still on board? Is this going to escalate to a hijack situation? Which agencies need to be involved? Who are the passengers? What nationality are they? What is the security at the departure airport like? All these questions are being processed while the aircraft is still in the air. If the Captain had unilaterally decided to divert to another airport there would have been even more chaos. Somebody said earlier in the thread that the Prime Minister was involved. He would have been. Several Cabinet ministered would have been gathered. The Commissioner of Police would begin forming a team. The SAS bleeps would have started and a team already deploying to PIK. This may sound like fantasy land or way over the top but this is what happens. The bill for this is huge. The man power is huge. A couple of hours delay for the Pax isn’t even in the equation.