The reason is that Lucasfilm decided that they would rather keep the mystery in tact for Plageuis backstory. Know only what they've told us and make us imagine the rest.

This and the fact that there is an untitled Star Wars book in the works right now that will be taking the place of the Plageuis novel. They stated that this new book helped prompt them to discard the other.

I know that doesn't help other than giving you a reason. I wish they'd reconsider, but at this time, they have absolutely no plans to further that story.

The reason is that Lucasfilm decided that they would rather keep the mystery in tact for Plageuis backstory. Know only what they've told us and make us imagine the rest.

This and the fact that there is an untitled Star Wars book in the works right now that will be taking the place of the Plageuis novel. They stated that this new book helped prompt them to discard the other.

I know that doesn't help other than giving you a reason. I wish they'd reconsider, but at this time, they have absolutely no plans to further that story.

Perhaps the story of Darth Plagieus will have some part in the upcoming TV-series?

Sue Rostoni recently addressed this in a recent blog entry. Just like slow mentions above, it's basically to keep the Plagueis story mysterious. As much as I wanted to read the Plagueis novel, I understand why they stopped it, and also like the fact that it's a mystery. The films are very much characterized by this and other SW mysteries.

I also wonder if they might be keeping the Plagueis back story open so continuity is free for use in the TV shows. They often put EU lit on hold during movie production, so as not to interfere. I don't see why they wouldn't for TV too.