If I have counted correctly, your opponent gained 7 VP on his last turn (6 from bishoping, 1 from an estate) while you gained 1 on your last. He won 16 - 8, i.e. before both of you played your final turns it was 9-7 for him. You should've been able to somehow buy the last 2 remaining bishops and 2 estates (gain the third last bishop with university you had in hand) on your t20, no need to pile drive estates.