Because the election is so far away, most people aren't paying much attention to it it. So when they are contacted by a pollster, they respond to the name they are most familiar with. So, for the next month, Trump will get a bump in the polls, even if all the news about him is bad, because of the name recognition factor.
Bad news only impacts the outcome of an election if it is published immediately before the elections. So a guilty verdict in this trial won't impact the election much.
But that's why there are political campaigns. Joe Biden will respond after the political conventions with a barrage of publicity.