Us vistor visa denied before they even looked at my application.

I was shocked after my application was denied at the interview before the visa officer even looked at my file. I applied at the US embassy in London and when it was my turn for interview and I gave my passport and my supporting docs to the officer, he asked me if I visited any country before applying for US visa. My answer was “No, I wanted to visit another country but my visa was refused.”, immediately without even opening my file he said my us visa is not approved and I may try later, then, called the next person in the queue. Did not even let me explain anything.

Is being refused a visa from any country make me ineligible for US visitor visa? Or could it be that its because I did not mention any visa refusal on my US visa application. But then again, my visa refusal was granted after I had already submitted the US visa application. Because when I submitted the US application my biometric appointment was after 9 months from the submission date and in that time i applied for visitor visa for Canada which was refused. But how am I supposed to update the us visa application after i had already submitted it so that I could add this visa refusal information on the application?