Seven probably didn't want to ever try and 'forget' what happened to her. She probably instructed the crew to respect her wishes on that as well. She ceased
to be Annika the moment the Borg assimilated her, and even though she has kinda sorta "become" Annika again after she joins Voyager, she hasn't *really*.
Kinda like, um, the old Tarzan story. Just cause he's human and was brought up by apes doesn't mean he stops being an ape just because human 'civilization' rediscovered him. He's basically more ape than human. Or something.
In a coldly logical sense, I guess what I'm saying is that Seven "was" Annika, but even post-post-assimilation she "is" Seven Of Nine. Although maybe maybe she should have more accurately changed her title to 141 of 141 to reflect her new position in the 'collective' of Voyager's crew. Or even One Of One, given she's an individual now (and therefore the one-and-only member of her own singular 'collective').
Identity is a funny thing, ain't it.