Because Walkure has their own style that is different from Sheryl's. Specifically, Walkure seems to be more of an adaptation of Jamming Birds, from Macross 7. A tactical sound unit specifically created by the military (though I guess their still civilian and just contracted, in Delta). As to whether or not there are similarities between Sound Force / Sheryl/Ranka / Walkure, who knows. It all seems to be tied to some basic concepts (Spiritua/Fold Waves), so maybe they could have similar effects against the various "opponents" they've faced.
Exedore was just one of the holdovers from Macross to Macross 7. There is nothing specific that says they have to do stuff like that. Those two are just the two series that have any returning characters. In general, there is usually a decent gap in time between the various series on the scale of 10-30 years.