1) Donald Trump won’t be involved. The top ten debate is in grave danger of becoming the Trump show. Even top GOP candidates’ performances will be evaluated relative to Trump, making it harder for them to make a good impression on their own.
2) The second-tier debate will be less personality-driven, meaning it will more likely focus on substance, and on the single goal most important to GOP audiences: defeating Hillary Clinton. Republican voters like candidates who concentrate more on beating Democrats than bickering with each other.
3) A presidential debate is a big deal, no matter what time it is. Sure, the primetime session will have a larger audience, but Fox News has a big audience at 5 p.m., when the first debate happens. A good performance at 5 p.m. could well be worth more than a mediocre one at 9 p.m.