The Commission on Presidential Debates has decided that neither the Libertarian Party candidate nor his Green Party counterpart meet the threshold to be included in the first debate (15 percent in polling averages). This is good news for Hillary Clinton, who has had a rough couple of weeks and has seen her poll numbers dip. The Clinton campaign believes that part of her polling slide is due to young voters shifting to the Libertarians and Greens, so it has been trying to come up with strategies to win back those voters. Having a debate stage where the stark choice is between Clinton and Trump will help that effort, and also limit the ability of third- and fourth-party candidates to grow.
Beyond that, Donald Trump benefits from a crowded stage. All the debates he’s ever taken part in during the Republican primaries had more than two candidates on stage, which allowed Trump to avoid direct challenges to his policy knowledge as the other candidates mostly fought amongst themselves. It’s notable that when Trump did have the chance to debate just one other politician (in proposed debates with Ted Cruz and Bernie Sanders), he backed out. Clinton on the other hand is a seasoned debater, who has gone up against Barack Obama and Bernie Sanders. So Trump is facing a more difficult terrain than he might wish for.