Because it's not a huge plot spoiler. If she was going to be resurrected (which she wont be, but as a hypothetical situation), they wouldnt show scenes of her resurrection in the trailers. They'll tease at things, like Sephiroth, but they wouldnt show any HUGE plot things, like Tifa's death if it were planned. Not to mention that Tifa has been shown in Dirge of Cerebrus,

And, as TurkSlayer says, it isnt a flashback. It's a dream sequence or an event in Cloud's mind, as he feels extremely guilty over 'failing' to protect Aerith.