Writer Jeph Loeb was diverted from comic book projects because of his position as head of Marvel Television, and artist Tim Sale said that the amount of detail that fans expect regarding WWII was a time-consuming challenge.
According to ComicBookResources.com:
Loeb and Sale have worked together on many acclaimed miniseries over the years at both Marvel Comics and DC Comics, including "Batman: The Long Halloween" and "Daredevil: Yellow." In recent years, Loeb's comic book output has been limited due to his position as Head of Marvel Television.
The claim is backed up by this interview with Loeb and artist Time Sale, also by ComicBookResources.com:
LOEB: ...it was a matter of finding the time, which, obviously, on my end became more and more challenging as time went by. And then, on the flip side, it was maintaining the passion for the story we were telling — and that was also a very big challenge.
In the same interview, we also have the following explanation by Sale:
Tim, when I spoke with you at Emerald City Comicon last year, you said that much of it had actually been done. How much did you really have to do to complete it? And talk about your artistic process here, because it sounds like a lot of it was maybe done eight years ago — maybe a lot of it was done more recently?
SALE: No, no, it was more piecemeal than that. But at the time we talked, yeah, an awful lot of it was done. I don't remember, at that point, exactly how much. But because of various reasons, there were delays, so it just took a while. It was challenging to do.
Why was it challenging? What was challenging about this project?
SALE: Well, one of the things for me was that it took me a while to get my head around how much minute accuracy that so many people who follow World War II expect out of these kind of things. I would run into artists like Tony Harris who would die to do it because they knew every damn thing about it. I had to relax with the fact that that wasn't going to be what I was going to do.
So, apart from Loeb's duties and schedule, it seems that another reason for the delay is that Sale granted to ensure that they got all the historical details correct — apparently a very daunting task!