Evan is the most disappointing because ultimately box office is about account and money. You can underwhelm and still be financially successful. Grindhouse did this because since its budget was lower it need to earn almost nothing to be a failure. Evan's budget (how it got that high is beyond me- I don't think Bruce Almighty made that much) set itself up for failure. I don't know what the studio execs were thinking when they ok'd that much money for a family comedy, but someone should have slapped sense into them sooner on this movie. It is just too large of a hurdle for most movies.
As for Grindhouse (I haven't had the chance to see it) I'm guessing the two thing that hurt it the most was overall length (too long for people, too few showing for the theatres) and poor tracking (implied audience due to excessive film fan tracking, too cool for the mass audience).
At the end of the day, Grindhouse's slight box office misstep adds to its cult allure. Evan Almighty ends up just draining a studio cash dry, because there is no cult audience to embrace it.