The idea was to keep things secret, so they couldn't ask Twilight to write her a message without explaining why.However, there's another reason to not to try to recruit Sunset: Communication and time: In the same Issue, Stygian says that he didn't wanted to recruit Sunset Shimmer because he didn't wanted 'another mirror' involved.Considering that Tempest left Equestria approximately at Pipsqueak's age, she must have done it before she could ever participate in Nightmare Night, leaving her with few (if any) memories of the holiday to recall in adulthood.
In the Friendship is Magic episode 'Luna Eclipsed', Twilight asks Pipsqueak if he's having his first Nightmare Night ever since he moved from Trottingham to Ponyville, but he responds that is his first one at all. In Issue #2, Tempest Shadow expresses shock at the existence of 'Nightmare Night'.