As it turns out, Vince McMahon made the decision to announce The Undertaker vs. Brock Lesnar in the Hell in a Cell main event via video promo during the airing of WWE Night of Champions. Although WWE tries to keep the sign up and cancellation patterns of theWWE Network close to the vest, the usual pattern month to month is there are very few cancellations the week of a PPV and the biggest spike in cancellations during any given month is usually the night/morning after a PPV airs. If someone is planning to cancel, they're not going to do it right before a PPV is going to air that they've already paid for.
Ultimately it wouldn't matter as the day of their billing cycle is going to determine when they'd be billed next anyway, but most people don't think like that and so right after a PPV typically ends up being the day with the most cancellations. So Vince made the decision that he wanted that Hell in a Cell match announced during the PPV itself in hopes that some people who were planning to cancel would be excited by the announcement and hold off or change their minds.
It's doubtful the announcement made that big of a difference either way, but that was the rationale for the decision.