I haven't seen it yet, so I can't comment, but it's been annoying me on r/movies for a couple days that half of the submissions are people bemoaning the unwashed masses who can't appreciate Pacific Rim and went to see an Adam Sandler comedy instead.
At the end of the day, it's a Kaiju movie. A CGI Kaiju movie, but still a movie about giant monsters wreaking mass destruction. This type of flick hasn't done well in the US for decades. I don't see why it's that surprising that another one failed. Cloverfield is the only real exception I can think of, and that movie had other factors that contributed to it's mild success. People in the US don't dig on Godzilla movies.
Part of that probably has to do with the way people here generally think about movies. Part of it probably has to do with there being two very big "fun" movies coming out at the same time. I think people are just kinda sick of watching shit blow up every summer. We've already had five other movies in the last couple months that are primarily about watching shit blow up. Maybe they're just burned out.
Of course, if you look at ticket sales, there's been a steady decline over the last 15 years or so. There were fewer tickets sold last year than since 1996, and the year before was even worse. This year looks to be lower than last year, and more in line with 2010, as far as gross, which means that ticket sales are probably lower than that as well. People are fucking burned out on this shit, and no one is picking up on that. They don't want to see it go bigger and crazier.
Ten years ago you'd have maybe one or two big movies from each studio in the summer. Now every studio has a new $100 M or more movie coming out every other weekend. Meanwhile, ticket sales are going down every year. So, yeah, obviously you're going to have more huge bombs. Fewer people are seeing movies, more movies are being released every year, and those movies are costing more and more. 20 years ago something like Pacific Rim would have been a huge hit purely for the spectacle of it. Now it's just another big stupid movie and we get twenty of them every summer.