Why's it rated so low?

You have to realize how stupid a wide margin of IMDB's visitors are, (424 people have given Deuce Bigalow: European Gigolo a 10, meanwhile 626 people have given Rashomon a 1) also considering the subject matter, I'm willing to bet a lot of people rating it low haven't even seen it and just give it a low rating because of racism and prejudice. This is the kind of movie like American History X or Trainspotting where, depending on how you want to see it (Pro Neo-Nazi, Pro-Drugs, Pro-Terrorism) is how you'll see it.

But if you look at the percentages, 31% gave it a ten, more than any other percentage. I definitely thought it was a 10.

Welcome to the bottomless pit.

Because it's rather slow and boring. There's no twists, nothing exciting happens and it even ends on a sad note with little meaning or purpose. There's no enjoyment or fun to be had from this movie. It's just a straight story from start to finish about a man preparing for a terrorist act. However, it is, at least, well done with nothing stupid or outrageous in it. Therefore it should be rated at about 6 or 7.

Rated it a 9 on first viewing 6-7 years ago. On re-watching recently would move my vote down a couple notches. Still a valuable insight when one lines up all post 9/11 movies referencing the "war on terror".
My big surprise on re-watch was seeing Mr. Shark Tank himself (Mark Cuban) listed as one of the producers. Guess not all investments pan out, would've loved to sit in on that elevator pitch!