"Biden remains the top contender among Democrats, however, and Trump’s lead over the other GOP contenders widened following his federal indictment earlier this month."
Given that one is the clear favorite among Democrats, and the other is the favorite among Republicans, it'd be perverse if we didn't get a rematch between them, unless that changes between now and the election.
This is how it is supposed to work in a two-party system. The favorites of each party get to compete. If there were somebody else who was the favorite of their party, they'd get to compete.
If it were a different system (e.g. a parliamentary system), then the rules would be different. Although tbh when I look across the pond to the UK, I can't say they consistently do a better job picking leaders than we do.