Just like no one mentions that Anakin is Darth Vader and Luke's father until near the end of the movie EP VI. Maybe it was easier to tell of Darth Vader's exploits as a conqueror than the hero of the Republic as he's killed so many before changing sides at the end. And Luke as the hero is easier as he's a Jedi.

We all paint history differently to suit our needs. If Ben/Ren would've known that Vader was good, it would totally change his view.

Vader never knowingly tried to kill his family. He just killed Jedi trainers/council/kids as his view was distorted. Ben/Ren's story is different in this regard.

Don't know why a heroine is the point of this story now (maybe need to vary it) as Leia never seemed to have learned how to use her Jedi powers as she's Luke's twin (therefore, equally powerful).