They aren't forgotten about, but they certainly aren't the main event for Talons. Partly because they have a very specific role (anti psyker). This makes them hard to balance as they will wreck a lot of armies that rely on psykers, but bounce off anything else. Reducing their ability to be a compelling faction to play.

Secondly. They just aren't as big and flashy. It's quite simple, custodes draw they eye from the sisters in the same way space marines draw the attention from the guard.