If the casual audience would've loved it enough, then the movie would've done a lot better in the US. They can spread the word all over the globe which would lead to a bigger succes, but apparently not enough people loved it. No doubt that there are people that enjoyed the movie while being unfamiliar with the games, but they seem to be to few in numbers. I've only seen real Warcraft fans who go out of their way to rate it an 8 or higher, who rate it out of enjoyment rather than how the movie is constructed.
The US flop could mostly be contributed by the poor reviews (from unprofessional critics) and awful marketing, not sure if a good word from the general audience could save it though.