Maybe because it's not actually very good? It's extremely partisan and conspiratorial, and doesn't actually document or explain much of anything, beyond "Bush bad". It's not surprising that a "Bush bad" film did very well in the aftermath of the Iraq War and the brazen lies from the Bush administration to sell it, but that doesn't mean it's actually a good documentary, or that it has lasting value.

I watched it back in 2004 and thought it was just brilliant, but I watched it again last year and I found it shocking just how incredibly conspiratorial and superficial it is. I do not consider it to be a serious documentary.