It basically came down to blackmail and Trump thought he could bully him.
Remember, zelensky stood up to trump over his withholding of legislated aid to Ukraine which resulted in trumps impeachment.
They gave me er been friends and trumps hate boiled over and he lost.
Let's say zelensky loses the war. Putin gets the spoils and Trump gets nothing. It could be Trump has to defend Ukraine for that reason, deal or not.
The aid given to Ukraine was legislated so is under no obligation to be repaid in minerals or otherwise. It's blackmail from Trump and zelensky basically called him out in it.
Trump will not push this guy around.