It’s not the respective rosters, though SmackDown Live boasts a more interesting group on the whole than Raw, at least to this writer’s tastes. It’s not the authority figures, which is basically a push. It’s not even the booking or even the creative direction, as it’s the same company and even when it’s a different popsicle it’s still flavored ice.
It’s this (both screen grabs via WWE.com):
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The Raw preview is generic and decidedly vague, with no concrete plans in place as of this writing. This is for the go home show to that brand’s next pay-per-view.
The SmackDown preview, meanwhile, is allowed to go in depth because damn near the entire show is already booked, with John Cena’s return, a women’s championship match, and a battle royal to determine the top contender to the United States title.
Yes, WWE has more motivation to announce the SmackDown card ahead of time because it’s a holiday and they need heavy promotion to avoid a disastrous rating (which is likely inevitable) but this isn’t the exception, it’s the norm. Each week, you can be reasonably certain the blue brand will tell you what you can expect next week while the flagship show will often keep you guessing all the way up until the last minute.