In Hollywood there is often little regard for truth, and much more regard for attention and money. The desire was to not show forth the glory of the Creator God or Biblical truth, it was to use a familiar story to make a movie along the line of Lord of the Rings. If you approach something as a myth...what's the big deal if you throw a few more myths into it? It seems that even holy scripture is at the whim of artistic license...but if one does not view it as holy scripture then there is nothing to hold you back from making changes.
The last thing I saw that even came close to following the Biblical text was the Passion of the Christ. It did have a little bit of "Catholicy" stuff in it, but it was...overall...well done.
It would be folly to approach the Biblical flood story as anything but myth. Like any good myth, the flood story illustrates important truths, and we do the story an injustice by treating it as literal history.