Because the Obama administration, which Biden is linked to, pulled out of Iraq. Obama criticized Clinton for her support of the Iraq war. Iraq was not an issue for Biden
Plus, Trump has been president for 4 years. He was the incumbent. He has to run on his record more than attacking his opponent. Calling Biden out on Iraq would suggest the US is still dealing with the consquences of Iraq, which would make Trump look bad and incapable of fixing it.
Yet instead of running on, “Here are my accomplishments”, Trump ran on “Biden has dimentia and wants to defund the police.”