So I've been musing on WHY Voice of Isengard might be delayed and basically everything comes down to either production or design. Under production I see two possibilities: 1) They literally can't get enough production time from their supplier because of some other games that are coming out. 2) They can't get all of the art work done in time due to their large art needs.
Or design... This is where it gets interesting. Since the Doomed player cards were spoiled I've been worried about the mechanic. For one thing, I've been using Strider all through the Gondor cycle and thought I would be moving on to a new hero for Rohan, but with that mechanic Strider becomes SO powerful it's crazy. Everybody will want him on game night. (And with the ruling that his ability can be used once for each player in the game it borders on game breaking.) So that's kind of where I think the issue really lies. I've a feeling that they found something in play testing that they felt they needed to address and so had to delay the release.
Obviously this is all pure speculation, but we have plenty of time to do it.