There's just not enough info to draw a conclusion. On one hand, SW was easier since it had the origin story which could fill pages. But also harder since maybe the archiving wasn't up to the level it would be. On Empire they knew people had an interest in everything so the Archiving and documenting would have gotten greater scrutiny, making production easier. But conversy, maybe finding interesting storylines from the production would be hard to come by, since people knew what to do and just got to it. So what could be holding it up are new interview material. Finding all the production people, chatting them up and seeing what fleshes out into printable material. It is a tremendous undertaking, first seeing/hearing what's out there, then putting it all together.
In the mean time, here's an interview with Roger Christian (production designer SW, Alien)
There's some talk about Empire in it.