Why is Iran unwilling to talk? Because they don't want to be coerced.

Why did Trump agree to delay? Because he's afraid of fighting on two fronts!
The US just attacked an Iranian ship and imposed a naval blockade on Iran. This is a tough move, very difficult to counter, and Trump also said that after a ceasefire, bombing would continue. If Iran agrees to talk at this point, their momentum would weaken; they wouldn't be stepping onto a "negotiation table," but a "surrender table."
The US side is likely to make some harsh demands, and even if Iran agrees to talk, they probably can't accept them. It's better not to talk at all.
If the US wants to act, whether they talk or not, they will strike; negotiations are just a "delay tactic."
It seems that the hardliners in Iran are gaining the upper hand, having no illusions about the US, preparing for battle, letting the outcome speak for itself.
These people believe that only by fighting can Iran force the US to accept equal negotiations and find a way to survive.
Currently, this is the best time to fight—US military preparations are the worst, markets are most sensitive, and they could deliver a deterrent result.
For example, oil prices hitting $200 or a collapse of the US stock market would be ideal.
Trump probably lacks confidence in starting a war, not because he's afraid of defeat, but because he's afraid of fighting on two fronts!
On one hand, he wants to win a glorious victory without shame; on the other, he needs to lower oil prices and support the stock market.
This task is very difficult—it's unlikely to start a war and have the capital markets stay unaffected.
He can only keep preparing and watching to see if an opportunity arises.
For example, Iran's internal unrest or economic collapse due to blockade might make them unable to hold out and force negotiations.
Iran needs to hold on, fight back, withstand the blockade, and find ways to sustain life and wait for the right moment, even if oil can't be sold.
Meanwhile, the US keeps deploying troops, trying to control oil prices, and possibly opening and closing the Strait of Hormuz, but things won't end quickly.
It might be a prolonged cycle of stopping and fighting.