My guess is that Watchmen's makers wanted to stay faithful to the work because they believed in it. My further guess is that they never came to an understanding of that work beyond a very facile take on some of its appealing surface aspects and genre correction elements. In that way, they're a lot like many of the book's fans. The result is a movie that while it's mostly faithful to its source material, the moments it's not faithful jar to a noteworthy degree, and the newly synthesized take of old and new elements never takes on a life of its own.
Read it all.